Right, to get Valable:

1. Install Vala
2. Install ctags
3. Get Valable: bzr branch lp:valable - this should give you a 'valable' directory.
4. Open Eclipse
5. Select File > Import... > General > Existing Projects into Workspace
6. Set root directory to the 'valable' directory from step 2.
7. Project should compile :-)
8. Right click on the project and select Run as > Eclipse application - a new Eclipse should open
9. Select File > New > Project... > Vala > Vala Project, enter some details, and create a new Vala file in it.

I've not yet looked at exporting the plugin for installation into Eclipse - it's a while off that yet. Let me know if this doesn't work.