I've recently tested out the most recent versions of 3 distros and if you use the built in package manager of linux to install eclipse you will get Eclipse 3.8 which doesn't have other eclipse projects included in it(In your case the WTP platform).
You can download and install by going to the "--All Available Sites--" option under Help Menu --> Install new software.
If it doesn't bring up anything your "Available software sites" list might be empty. Add the webtools update repository to the "Work with" dialog.(Cannot post the http here forum is preventing me with a silly error : You can only use links to eclipse.org sites while you have fewer than 5 messages.)
(I'm still a greenhorn at this, so if there is a mistake with the above please correct me.)
Just follow the guide (pm me for the guide forum preventing me from posting links -.-) to get the latest eclipse (Eclipse Juno 4.2) with all the perspectives and development platforms preconfigured.