Have you enabled "Media Server Connection" in Network Settings on your PS3 XMB.
If others are recommending other streaming apps.........
I will recommend PS3 Media Server.
http://ps3mediaserver.googlecode.com...s-1.20.412.exe
If you want to be able to stream Hi-def MKV (my copy of Avatar is 19GB),
I would not recommend Tversity. Especially MKV with subtitles.
There is a way to get it to work with Tversity, but it involves tweaking codecs and such. I have zero codecs in my PC and have flawless streaming of all but 5 of my 500+ movies of varying formats.
PS3MS has real-time updating of shared folders......Tversity has to be scheduled to do folder scans or you can update your folders manually, but large libraries take forever to scan.
Just my opinion as I have run both at the same time for comparison, and quickly removed Tversity after that.
Windows media player for the PS3 is awful in comparison to both Tversity and PS3 Media Server.