can't get cube Lexicon to energise need code???

You need to speak with Septimus Signus, then he opens it.

I think the OP is about the process in Tower Mzark. You place the cube, do the incantation to open up the Elder Scroll container, and then take the cube back (it's now full). I don't remember any message that tells you that directly, you just get the "take the cube back to Septimus" one.