Try using AppCleaner instead. Open up AppCleaner and drag MacKeeper into the AC window. It'll then automatically add any related files related to MacKeeper that has to be deleted.
If you elect to use such apps, be aware that in most cases, app removal software doesn't do a thorough job of finding and removing files/folders related to deleted apps. For more information, read this and this. If you just want to delete the app, drag the .app file to the trash. No other software needed. If you want to completely remove all associated files/folders, no removal apps will do the job.