Hey Vivian
When looking for Beoport on eBay, do also look for "Beolink PC 2" (must be PC 2, the older Beolink PC would not work). That is the old name for the box.
Beoport is a HW box that connects via Masterlink to your Ouverture and via headphone and USB cable to your powerbook. B&O does not offer software for the Mac - for which I am quite happy given the stability of the BeoPlayer for Windows. So you will have to load Anders' Ödlund tool "BM-Link" on your Powerbook (http://www.odlund.se/bmlink/) to be able to control iTunes via Beoport.
Once everything is installed, you switch your Ouverture to N.MUSIC (you will have to add the command on your Beo4), which causes the Powerbook to launch iTunes and play the selected songs. You can then jump from one song to the next, back etc., basically everything you would expect from controlling the CD-player.
If you spend some additional money on an IR-eye for the Beoport, you can also control FrontRow with it. There is one drawback, however: To be able to address the Beoport IR-eye, you will have to switch your Beo4 to PC. For my taste this makes the thing too complicated: First switcht to N.MUSIC to start listening to iTunes, then switch to PC to control Front Row ... I'd rather control iTunes directly, without engaging Front Row.
Also, you may want to add the command N.RADIO to Beo4. This would allow you to listen to Netradio. The preferred stations would have to be grouped in a playlist called N.RADIO.
Apart from not having to buy and install Airport, the thing I like most is that I can control iTunes from any room in the house where I have a linked B&O device present.
Hope this helps
Best regards
Stefan