Not a BeoSound 5 user myself, but it seems as if your dealer isn't correct. You can store music in WAV format, and play that back. The disadvantage is that WAV is uncompressed LPCM (such as found on a CD), and therefore really eats hardware space and processing capability. Apple Lossless is identical to the CD, but compressed compared to WAV and from 40-60% of the file size of the latter.
For some reason, B&O chose Windows XPE with a skin for BeoSound 5 (previously having used embedded Linux), which is why BeoSound 5 is heavily tilted towards the Windows universe of compatible formats.

Here are the supported formats: WMA, WMA lossless, MP3, WAV, ASF, AAC

BTW - while one will hunt both high and low for good spec's in the user guide or on the website, here's a nice link:

link

Windows users who need a good program that will hunt for album art efficiently might like this one:

link

I have a mac mini and a macbook. Both Intels. So via Parallels/XP I can use the Beoplayer 5.00 for my BS5. Works great. 

For my itunes music I have 320-AACs. I think this is a very very good format, I can hardly hear the difference to the original, only sometimes with classical music.

To transfer music and playlists from mac to BS5 you can also use Beoconnect.

Stefan