Thanks guys, you found a bug in mintInstall
mintInstall checks to see if the package is installed (for instance.. skype). But at the stage, before you click install, it's still using YOUR repositories.. not its own.. so if the package is not in the repository, it fails miserably
Use a fallback on the call to the cache. I'll fix that very soon.
The reason you don't have skype in your cache is because you're pointing at the old address for medibuntu. Update your /etc/apt/sources.list: