I tried unchecking and re-checking the "Contact all update sites during install" option, but I didn't exit Eclipse in-between like dprokosch did, so that didn't help.However, as per advice from mrabbi, I examined the associated sites xml file, and then looked for them in my "Available Software Sites". They were there, but they were not checked. So I checked them, but still had a problem with missing the apache-xalan bundle. So I checked one other that seemed related, and they install finally worked! As dprokosch suggests, our Eclipse must be a bit messed up. The given associated sites were: http://download.eclipse.org/modeling/tmf/xtext/updates/composite/releases/http://download.eclipse.org/tools/orbit/downloads/drops/R20120526062928/repository/http://download.eclipse.org/technology/nebula/snapshotThe extra one I needed to check was one of these (I cannot remember exactly which one): http://download.eclipse.org/modeling/tmf/xtext/updates/http://download.eclipse.org/modeling/tmf/xtext/updates/Any way, it is working OK now. Many thanks mrabbi for the quick response.