Simplest way: get JBDS 5 from devstudio.jboss.com/earlyaccess.
That aside then as Denis says, make sure you have already updated to eclipse 3.7.2 before trying to install JBoss Tools.
You do that by either downloading latest eclipse 3.7 from eclipse.org OR run Help > Update.
Once that is done try use your downloaded archive and if that does not work let us know the exact error you get (its a long winding message with "p2" messages in them).