I have completed all of the thieves guild missions up to this point. But after I completed decoding Gallus's journal Karleigha told me to meet her at the ragged flaggen. But the mission " the persuit " does not come up. I really want to kill Mercer Frey. Any help would be appreciated

When you talk to her in the Ragged Flagon nothing happens? Or she isn't in the Ragged Flagon at all?

I recommend going through the http://www.uesp.net/wiki/Skyrim:The_Pursuit#Bugs to see if someone encountered a similar problem. You should also try to reload a previous save if you have one.