The fact that it doesn't show up in NEI is the proof you modified something.
I use the Mindcrack pack 8.0.1 with no modifications, and I do have the recipe for the diamond block in the compressor.
Let me check the config files I have and edit my message.
EDIT:
I think I found what could be wrong.
In RecipesAdvancedConfig, lines 215 and 231.
These are lines that permit the recipes, apparently, or at least partly, because making just the line 215 to false (default being true) makes the recipe for diamond blocks out of diamonds disapear. It keeps the one from industrial diamonds (line 231).
The line 1125 set to false also disables both recipes.
So in order for the compressor to have the recipe, you need both the line 1125 to true, and the line 215 (optionally the line 231 too for industrial diamonds to diamond blocks).
I hope it helps you in solving your config problems.
If all fails, you can always backup your save and configs and re download the pack fresh too, it usually helps in such trouble shooting.