Your two Sims are somewhere in vacation and considered as guests ? Have you installed the update for Bon Voyage found on the official site ?
I would use the Insimenator to make them return at their house in university.
It's normal that your computer can't open .package files (you need SimPe or other program). The sites where you download files explain surely what to do with them but ok. :-P
Open your My Documents / EA Games / The Sims 2 and search a Downloads folder. If there isn't any create yourself one folder writed correctly
Downloads with the "D" and the "s".
Place the package in it.
Load the game and allow the custom content. Go in a family. Desactivate the window on the custom content in the options and save. Go out of the game. Load again, the custom content must be integrated now.
If you have downloaded the Insimenator click somewhere and spawn the "summoner". Click on the box you will have the option to "restore the family". See
there.