On any Mac OS since 10.3 you should be able to just enter the WPA passphrase and it will know that you're joining a WPA network... most of the time on Macs you don't need to tell it to use WEP or WPA, it can auto-sense that info from the signal.
Just go up to the wireless indicator on the menu bar, click on your company's wireless network (if the SSID isn't being broadcast you might have to click "Other..." and enter the SSID), then enter the WPA-PSA-TKIP passphrase into the password field and click OK.
Good luck!
Eps.