So I installed the UC on my Mac. Was happy and installed the sim package editor thing that came with it so I could download lots and sims.
But recently the stupid thing has stopped working. It won't let me install sims or lots.
It says I installed a new expansion pack which is ridiculous.
Any ideas or anyone about who knows a way around this problem?

Well I ran in to some problems myself. Try taking the download file out, starting it, then shut it down, then put the file back in, sometimes it seems to confuse itself. I had to do that and make sure you have it enabled... I forgot to enable my downloads once and finally figured out that was the problem/

I don't use a Mac myself so I hadn't even heard of this Package Installer thing (at first I thought you were talking about Clean Installer, but I'm pretty sure that's Windows-only), but in this discussion on MTS, the posters are talking about how to install CC on a Mac without the Package Installer. There are several people with different (related) problems, so they discuss it from several different angles. Maybe that would be an acceptable workaround if it works. Good luck!