After the questionnaire, you'll have to upload a text file with system information about your Mac. To save your System Profile information, go to the Apple Menu > About This Mac > More Info, then select View > Mini and File > Save. Valve says the file should be a 20-30kb UTF-16 TXT file, and if it's larger than that you should make sure it's not saved as an RTF or XML.
The company notes that it is a closed beta, and participants will be "selected to provide a broad coverage of testing criteria." If you're not chosen, don't fret, the Mac client is due to launch sometime next month.