Yes, I have done it.
Take the .zip. Extract it into a folder on your phone. Somewhere accessible, like /home/user/MyDocs (root folder when you attach phone in mass storage mode; N900 (or something like that) when you use default File Manager on the phone).
Now you see four files in File Manager on the phone. Tap one of them (.deb; its icon is white-and-red), and Application Manager will start. Agree to install Pac-man slider. When it says "application successfully installed", it's installed. You can see Pac-man background. But the slider isn't reloaded it; you need to reboot.
After un-installation, you will also need a reboot to get default theme slider back.

Warning and disclaimer. Theoretically, not every theme works will with it (though most do work as intended). It was tested only with default theme on Maemo 5.
And please, if you installed theme while using theme x, uninstall Pac-man first before uninstalling theme x or updating it; and when uninstalling Pac-man, you MUST use exactly the same theme you used while installing Pac-man. In the worst case, you will lose lock-slider images in one of your themes.
In short, don't do anything with themes while Pac-man is installed, or you might scratch your head in thoughts about lock-slider chaos. If you follow this rule, and theme is standard (has the lock-slider images named standard way), everything is fine.