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.