NOTE: Mesh is not available in Phoenix 1185 or earlier.
Phoenix supports rendering (display) and upload of mesh. Rendering of mesh requires that you have two options enabled in Preferences → Graphics: Basic Shaders and Hardware Skinning.
To test the display of mesh you can go to Mesh Mellows; there is a TP near where you land which will take you to the mesh gallery.
Mesh upload is done from the top menu, File → Upload → Model.
For more information on mesh, refer to the SL Wiki:
If you have problems seeing mesh, please refer to this page.