Yes, you will need a DB25 cable. It would need to be either DB25 to TRS, or DB25 to XLR, depending on what kind of inputs are on your compressor and EQ. From the compressor or EQ, you would then go out from those units with a regular TRS to TRS, or XLR to TRS, again, depending on what jacks are in those units.
The TRS cable would then come back into line inputs 1, 2, 3, or 4, whatever you choose.
The thing to remember when doing inserts for mixing is that whatever output number it comes out of, it needs to come back in the same input. So if you use output 3, you come back into input 3. In the Protools software, on whatever track you want to use it on, you select an insert just like when you're inserting a plugin, but instead of "plugin", you select "I/O", then choose which input you hooked it up to. And that's it. Now the hardware EQ or Compressor is inserted into the track just like a plugin would be and you can use it the same way.
There may also be some adjustment for latency because of it being a hardware input/output, or it may be adjusted automatically. Someone with more experience could chime in here.
To be more specific, the DB25 cable will have numbers on each of the cables as they go to TRS or XLR, etc. So you would choose the cable that says "3" for our example. Then from the Compressor or EQ, you come back in with a normal cable to the Omni input 3 jack.
Here is a good picture of a DB25 cable, you can enlarge it and look closely at it:
http://www.musiciansfriend.com/acces...rs-snake-cable
That's a good cable to get, by the way. You don't want to skimp on quality if you can avoid it, though many people are happy with cheaper brands like Hosa, etc. As mentioned, you may need the one that breaks out to XLR instead, depending on what input jacks are on your compressor/EQ.
Note that this is all for mixing, because that's what you asked about. If you want to
record/track through the units, that's a little different, and easier. Let me know and I can explain that as well.