Moving this discussion to the Photoshop Elements forum.
For plug-ins, different versions of PSE don't see each other's installed plug-ins, but you can point PSE 10 to where you've got it installed for PSE 9. In the editor's preferences>Plug-ins, just turn on the option for an additional plugins folder and point to that.
What's the problem with bridge? You can't launch it from within PSE 10 but it should still work just fine and be able to send photos to 10 for editing.
To add to Barbara's suggestion, you'll need to check the OnOne site to ensure your version of the plug-in is compatible with PSE10. If not, you'll need to get the OnOne version that is.
Yeah, very important point, Ken, thanks.