portal glitch - Help?

I can't see through a portal. Instead, I just see blackness. It's only just started happening, so I am really confuses. I'm on a mac, and I downloaded portal from steam. What gives?

humm, maby if you go through it youl find some cake ^_^ yay fir portal fever

Try this:
http://forums.steampowered.com/forum...52&postcount=2

Thanks a ton!