That looks like the plugin is not properly installed.
[Guiri Reyes]"tryed to uninstall , but wasn not able to do it, found the folder that have to be deleted and everything...
tryed to install twixtor 5 btu it didnt even show up on FCP."
Go to Main/Library/Applications Support/ Final Cut Pro Support/ Plugins.
Delete the plugin from there.
You may need to delete the Twixtor Preferences as well: Main/ Library/ Preferences/ REVIsion Effects/ Twixtor.
Also you may need to delete the REVision folder on the Applications, if any.