What's the assertion? Are you running under Mac OS 9 or under Mac OS X?
As for saving, there are two things that can cause regular save game failures. The first should be true for ToB under any version of Mac OS or Mac OS X, and the second is only applicable to Mac OS X 10.4 or greater:
In the Baldur's Gate II folder, there might be a file named "temp" -- if so, delete this file (drag it to the trash) and create a new folder (Cmd-Shift-N). Name the folder "temp" (without the quotes, of course).
You also need to exclude the Baldur's Gate II folder from Spotlight (its indexing can interfere with the way saving the game is handled). Launch System Preferences, click the Spotlight icon (in the Personal section), select the Privacy tab, and add the Baldur's Gate II folder to the exclusion list (either by dragging the folder onto the list or by pressing the '+' button and navigating to and selecting it in the resulting dialog sheet).