Thats because you want to use svn as the scm protocol, there should be a drop down menu with a list of options.
You have to build the project using maven.
You can just checkout the project in eclipse from svn if you want, you can download the source zip and use that (either by importing it as a maven project or as an existing project). I like the way that eclipse now handles maven projects and find it to have less issues.
I have updated the maven pluging, I now have the dropdown, but the result is the same.
it sounds like you don't have the plugins installed properly. You might need to ask the maven eclipse people about this.
Or you may want to just checkout the project as an svn project (the .project and .classpath files should be committed in svn).
the proper url for scm checkout appears to be
scm:svn:http://anonsvn.jboss.org/repos/gatein/portal/trunk
this gives me a collection of exo projects in eclipse. However, the errors remain. There must be some setting in Eclipse I'm missing that cause all dependencies to remain unresolved. Like that the "maven dependencies" folder only contains three jar files: two for slf4j, one for hsqldb.