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If you are using a downloaded version of uPortal you should simply be able to add the uPortal project to your Eclipse workspace using the "File > Import..." command and selecting "Existing Projects into Workspace". Select the uPortal folder which contains the .classpath and .project files for Eclipse.
An [An online video|http://www.rci.rutgers.edu/~faizan/downloads/downloads.html] from Faizan Ahmed from Rutgers University is a helpful demonstration for importing uPortal into the Eclipse IDE
Building uPortal
Eclipse ships with Ant 1.7.0 which has a known bug that does not allow it to work with uPortal's build.xml. You can either use the command line to run the uPortal Ant tasks using Ant 1.6.5 or go into the "Ant > Runtime" section of the Eclipse preferences and select an Ant 1.6.5 distribution, this will allow you to run the uPortal Ant tasks from within Eclipse.