I found this on the Treo Central Forum. If you have problems, go there, read the thread and see if someone had run across your problem before.
First if you have TCPMP installed on your phone remove it.
You will need two files
TCPMP Smartphone version
Flash Video bundle ( You need a free TreoCentral.com account to download the file. )
Both files must be installed on either the card or phone.
Do a soft reboot on your phone.
Go to YouTube.com. When it redirects you to the mobile site go to the bottom of the page and click the link to ‘classic view’ and you’ll return to the main YouTube site. This plugin does NOT work on the m.youtube.com mobile site.
Pocket PC Freeware also has this software TCPMP & Flash Plugin for Windows Mobile. I didn’t try the versions there. I see no reason that they wouldn’t work just as well.
The TCPMP player will also let you listen to streaming music.
If for some reason you still don’t get it working try XDA Developers YouTube Tutorial