The current version of the GPD is painlessly working in combination with Eclipse 3.1 and WTP 0.7 with its required drivers. No doubt MyEclipse is using another version/combination and thus a configuration that is not supported. Best thing to do when working with the GPD is IMO to use a clean Eclipse 3.1 or to use the complete JBoss IDE offering.
I had the same trouble when using MyEclipseIDE.
1st: Install eclipse3.1
3rd:install jbpm Process Designer
4th: reinstall JBOSSIDE (download from jboss.org)
this works fine!