Nick,
After setting your proxy at Window->Preferences->General->Network Connections, are you able to access any Web Site, from the internal web browser (which usually appears in the bottom central panel but can be started via (Window->Show View->Web Browser)? If not, your proxy settings may not be accurate.
Check the .log file in the .metadata folder of the workspace. If there are messages of the form, “System property http.proxyHost is not set but should be 192.168.5.102.” then you may need to restart MyEclipse or add extra options to the myeclipseforspring.ini file (in the same directory as the myeclipseforspring.exe file) something like:
-Dhttp.proxyPort=8080
-Dhttp.proxyHost=XXX
-Dhttp.proxyUser=XXX
-Dhttp.proxyPassword=XXX
-Dhttp.nonProxyHosts=localhost|127.0.0.1
This seems to be an issue with some eclipse installations.
If this doesn’t work, then you may be able to install the plugin via a downloaded update site (check the plugin home page) or via a downloaded plugin dropped in the dropins folder.
Please let us know if this helps.