have you successfully installed anything else before? Did you try installing a different app (any random app will do) to make sure its not just a problem with the specific app you are looking at?
It works with other apps but it wont work with about one or so. I can install the extended packages and it'll show up in the unistall area but it wont be installed any wheres else. It's suppose to be in the tweaks were you can install it but then nothing is there so I went to check int he General area and there is nothing in there. I uninstalled it and then tried it again and still the same thing.
I have version 3 installed and I did what you said and still nothing.. I got rid of the Extended url and I still have the package installed... I can't uninstall it or nothing so I don't know I'm stumped
If it is just the one app (ie other apps install correctly) there is probably something wrong with the source or just the app itself. If the app has worked before, try tomorrow or something. I have not had this error before, but there have been days where one app just wouldnt download/install correctly, but it worked fine the second day I tried it. Just be patient. Let me know how it goes trying later.
Also, let me know what app it is. I will try and install. if it works, well we can talk. If not, then we know there is nothing we can do unless it is fixed by the developer.