If you are using SMF 2.0.x, and have had issues during installation or uninstallation, and a package looks installed when it isn't, or uninstalled when it is installed, you can use this tool to fix it: Fix Packages.

Copy this file to your SMF forum main directory, where SSI.php and Settings.php is located. Then point your browser to the file fix_packages.php in the same directory as your forum. Select the package and change it to installed or uninstalled.

Do not click the Remove all hooks button unless you know for sure all your mods are uninstalled.

Afterwards, with the installation status set properly, you can attempt to install/uninstall the mod again.