I took mercers plans before I should've. After Enthir decoded the journal karliah just
Left and I can't find her anywhere. Please help.

Put the plans back in Riftweld Manor and then, in the console, input: setstage tg07 10.

This will give you the quest. Karliah will be at the entrance to the Ragged Flagon, on the Ratway side. You have to appraoch her from that entrance in order to initiate conversation. (though you can enter from any other location.)