RESPONSE: Excellent question. By clarifying a couple of words, I think the answer will become obvious. The words are everywhere and presence. First of all, what do we mean when we say that God is everywhere? Since God is spirit (Jn 4:24), He does not have a body.* Therefore, to say that God is everywhere does not mean that He is spatially extended in all directions. God is not a cosmic Gumby or Stretch Armstrong. So, what does it mean? The best way that I know to say it is that God is consciously aware of, and causally active in, all places. Not only does He have power to act in all places, but He also sustains the entire universe in existence (Col 1:17). So, is God "in Hell?" Yes and no. Yes in the sense that God is just as consciously aware of, and causally active there as everywhere else. No in the sense that God is not physically present in Hell, but that runs us into the other part of the question which deals with God's presence.
If God is everywhere, what do Christians mean when they say that Hell is separation from God's presence? Unfortunately, even though the phrase God's presence is typically used by Christians in a highly nuanced way, it is rarely explained. Since God is spirit, to be in God's presence is not physical relationship but a spiritual one. Furthermore, since God is spiritually present everywhere, then it would seem it is impossible to be separated from His presence--that is the crux of the question. However, when Christians typically speak of being "in God's presence," they are referring to something else altogether. They are talking about experiencing communion with God--connecting with Him.** Otherwise it would be nonsense for a Christian to say something like, "God really showed up," or "God was with me today." Now, personally, I do not particularly like these phrases because I think they can create confusion (as seen in this question). Even so, they are perfectly acceptable inasmuch as they are not meant to communicate the actual presence of God, but a connection with Him. And it is in that sense that one can speak of Hell as being separation from God's presence--spiritually disconnected from communion with Him.
QUESTION: Is Satan in Hell right now? If not, where is he?
RESPONSE: That depends on what one means by "in Hell." He has not yet been cast into the lake of fire (Rev 20:10). That will be His final state, and is what many people mean when they refer to Hell.*** In that sense, the devil is not in Hell--yet. However, his sin has eternally separated him from the purpose for which he was created--a love relationship with God. In that sense, he is as much in Hell (a hellish existence) as one who has died without Christ. But unlike those who die apart from Christ, the Bible indicates that he (and at least some other fallen angels) are free to act in the spiritual realm, waging war against the Kingdom of Heaven--which directly leads him to wreak havoc on the hearts of men. The Bible says that he roams about like a lion (1 Pt 5:8). He is called the prince of the power of the air (Eph 2:2). He has been given dominion over this world/age (2 Cor 4:4). However, the Bible is also clear that he is subject to the sovereign purposes of God (most clearly seen in the story of Job). As it has been said, he has only been given enough rope to hang himself.
*I am referring here to the Divine essence, the Godhead, the Trinity--Father, Son, Holy Spirit. Of course, the second person of the Trinity became human (while remaining Divine) in the person of Jesus of Nazareth, and the God-man incarnation is eternal. That is, the Son has a body. In fact, Jesus has the first, resurrected and glorified body--He is the "firstfruits" of all the redeemed (1 Cor 15:20).
**To be sure, there are accounts in Scripture of God's presence being manifest in physical ways such as Genesis 18, Isaiah 6 and Exodus 33. But we must remember that these are condescension on God's part. That is, He is appearing in ways that we can comprehend.
***A similar discussion can be had with regards to what it means to be "in heaven." The souls of the departed redeemed dwell presently in the glory of the Lord, but final state of the redeemed will be to dwell in glorified bodies in the new heavens and earth.