I downloaded the WSO2 eclipse plug-in (developer studio) 3.2.0. I tried to follow the "quick start" http://docs.wso2.org/wiki/display/BPS300/Quick+Start+Guide for BMP and got stuck at point 18. It is not possible to edit the expression in the properties window of an Assign Activity. I tried with Eclipse Juno, Kepler and even an older version of Eclipse, all with the same result.
Sometimes an box appears under the word "expression", sometimes it disappears but it is never editable.
