IIRC some changes have been made to JBossTools-Forge that seem to prevent it from being installed in JBDS 7.1.1.GA or earlier. It installs fine in JBDS 8.0 Beta1 though.
As to sorting out the startup issue, this error looks like a problem with the current nightly build (something to do with an incorrect version of Weld that was fixed last week). I'm afraid you'll need to build it from source (and you may need to go the extra mile to build Aesh and Aesh-extensions 0.53-SNAPSHOT since they're not in the JBoss Nexus repository).
I'll be the first to agree that it's a bit too much to ask of someone, so I prepared a custom update site that you could use - https://dl.dropboxusercontent.com/u/28645282/forge.site-1.3.0-SNAPSHOT.zip. Note that I'll be deleting this once the usual Forge update site works.