If you are a user of an older version of Max, your preferences will not be imported. Preferences include the search path additions you specified in the File Preferences window, as well as DSP settings and MIDI port mappings. You will need to recreate these settings. Saving your preferences will not overwrite those of older versions.
Max uses file extensions to determine the type of file you are trying to open. For example, when saving a Max patcher file, you must use the .maxpat extension or your file will not open properly.
Some Macintosh users of previous versions of Max may have been somewhat casual about file extensions, instead relying on the Macintosh OS's legacy "file type" mechanism to identify their files. This is no longer advisable. Furthermore, when importing old documents, Max pays more attention to the file extension that than it does to any Macintosh OS "file type" information that might be associated with the file. For example, if you have a text file and you have made the mistake of giving it the extension .mxb, Max will think it is a Max binary file until you change the extension.
This has always been the case on Windows; now it is the case on the Macintosh as well.