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.

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