i WAS experiencing this same problem.
no love on the network activation.
i prefer to use sub-directories, so i skipped step 2, as it says.
so... on a whim, i compared wp-config between the site i'm working on, and another site (not a network site).
i noticed that the define ('WPLANG', ''); was commented out.
so, for the heck of it, i commented it back in :D
re-uploaded my wp-config and presto! network mode!
not a clue why or how that worked.
or if it was in fact at all related...
but i have network under my tools menu now... and that's what i care about :D