Purchased base game..downloaded expansion won't install?

Awhile ago I purchased the base game, and for hours I have been attempting to install University Life but no matter what I do it always comes up with the ending message that it cannot install. I followed the steps of the Sims 3 Mac Guide. Do you think it has something to do with how I don't have the files of "Sims 3.app" is mentions, since my version is purchased?
Or maybe it has to do with where I'm trying to save it during this point:

And I've tried downloading the Ultimate Fix as well, but wasn't really
able to actually open it even though I downloaded the "Expander" the
guide tells you to download to open .rar files.

I don't know I'm completely lost. I'm just trying to make Sims actually enjoyable again instead of the bland base game. I'd greatly appreciate any help.

Hello,

Did you understand the context of that sentence: " Select a volume where The Sims 3 is already installed" means you should select the file where your Sims 3 actually installed. Usually is in Applications/The Sims 3 file. If your game application isn't under a file in Applications then create a new file and put your Sims 3 base in it and then name the file "The Sims 3" and put it back to Applications.

And whenever you want to install a expansion pack, please choose to Macintosh HD/Applications/The Sims 3 and click yes. It'll install it.

I tried that, even retried this morning, but the installation still fails. The message that says my version of The Sims "is not updatable and not compatible with University Life", has always came up too, but the guide said that I should just ignore that, correct?

When it says to that "updatable thing" can you continue to install or it force you to quit?

Then you need to update your game manually from
here. Just make sure you download the Mac version.

After downloading it, just open it let it patch your game up-to-date. Then try to install the expansion pack again.

Woo so it successfully installed! Still need to make sure the actual game play works though but thanks for your help! I appreciate it.

Hello,

You're welcome  I am here to help.

Please try the game and let me know the result. I'll keep the thread open for your further update.