Note: Instructions below are outdated. Will update soonish. For now, go to the Eclipse Market Place (under Help menu in eclipse). Search for "jactr" install the core and the IDE support. It will take some time, but the tool seems to (finally) find all the dependencies.
Assuming you have installed the prerequisite software, you are now ready to install jACT-R. The first thing that you need to do is to tell Eclipse which update sites to search for the software. Those who are less inclined to read can view either of these screencasts (install1, install2).
The update manager allows you to specify which websites to query for new or updated software. You're going to tell it about a series of update sites for the core, ide, and example software. (you can also import all the relevant update sites by importing this file)
