If you mean retail, then just plonk the Inf VP (or VP's
if you did the multiple download) into the Freespace 2 directory beside all the Volition VP's and it should load them.
EDIT: If you have any .tbl files in the "Tables" folder of "Freespace2/Data" then you will need to remove these
and place them in a backup file outside of the tables file
or the game will load them instead of the inferno VP and you'll end up with all the ships masqurading as Ulysses fighters
If you mean the Source Code Project FRED2 I don't know entirely about SCP since i don't have it, but i think you place the Inferno vp into a folder called "Inferno" in the MOD's directory and then use the -MOD command line and select it. Then it should work.
BTW: You require a patch to get Inferno R1 to work on SCP. I don't think not having the patch stops Inferno form working completly (it just crashes abruptly on some of the missions), but it might be worth getting it.