Summers, when you open the page where you find the save file, you can see the list of "mirrors" - these are sites where UC is is uploaded - just click on one of them, then download UnionCove, when file is saved, click "open" and it will start the launcher and install in game. Only then you can use the save file of it.
You can click on this link and chose slow download to get it:
http://www.carls-sims-3-guide.com/unioncove/download.php(Rapidshare is working without problems by selecting the middle one "Save file on your PC" by clicking on Download button in green square)
EDIT:
About lag, it is a huge world, and it takes time to load all objects on huge lots. We had some people on official site testing UC with great PC's and reported no lag at all (people that usually test worlds there). It is bigger than BridgePort (It does include all LN venues and Pets, and everything else game has to offer, which no EA town has). I managed to load it with highest settings today, but it is very hard to navigate around (with average processor and just 2GB of RAM). With settings on low, I can play it without lag and save without errors, as most of the others can, so I'm positive you'll be able to play around with it! Good luck and let me know if you need help with it!
EDIT 2: There are around 2 times more Sims and lots than in SV, but number of Sims will change with story progression on - Sims your PC can't handle will move out, but again, your PC is not bad at all - it is much better than mine and not much Sims would need to move out. Depending on your graphic card, only settings may be troubling, in terms of long loading - lowering them solved even the error 12 for me.