How to merge two mirrored vertexes? I have made half of my model and the lips look fine when I set Subsurf to be simple. But if I change it to Catmull-Clark then the lips get a gap in middle.

Simple:



Catmull-Clark:



I used a mirror modifier for the right side of the character.
All you need to do is move the mirror modifier up. It must be first in the modifier stack. Also make sure you have clipping ON (like it is in your pictures).