Reviving (?) this topic since i've run into the same problem, IC2 cables won't connect to the converter, assembler etc. and vice versa.
If it's because of a some other mod that is known for messing with OC and i've been too blind to see it anywhere, these are the ones i have installed right now:
Forge (obviously)
Armor Plus
ExtraUtils2
IndustrialCraft2
Iron Chest
Just Enough Items
Mantle
OpenComputers (hue)
Reap Mod
Redstone Paste
SlashBlade
Tinkers' Construct
Any help appreciated.