LaRowen,
Before we start, make sure Tequila is closed, and not on the taskbar.
goto your game directory before COH folder;
Example " C:\Program Files\Games\COH\" One in bold.
Right click on the COH Folder - (you will know if its the right folder as the cityofheroes.exe is inside)
And you should see something alike to this.
See the "Read only" bit thats ticked? Untick it, and relaunch Tequlia. Should load the manifest now =)
Give that a go, let me know how you get on =)