I have come across the same problem, I don't know why.
And even more, when I create BPMN processes by http://academic.signavio.com/p/explorer, and export the process and then import it into jbpm eclipse,
then eclipse (BPMN Visual Editor) throws out some exceptions.
At last, I find that [ dataobject can be attached to sequence flows in http://academic.signavio.com/p/explorer], but it can't be attached to seq flows in BPMN Visual Editor]
After I have correct all errors (just by deleting original DataObject and creaiting them and attaching them to some tasks )
I import the process into Guvnor,
then a strange thing takes place again: web-based designer in Guvnor shows nothing in its panel for the process.
Can anyone give some advice?
This is a known issue https://issues.jboss.org/browse/JBPM-3279 which was recently resolved.
The visual representation of lanes however is still to be done. With designer version 1.0.0.053 which you can get from sourceforge: http://sourceforge.net/projects/jbpm/files/designer/designer-1.0.0.053/
you can set the lane names in the process properties (see Lanes property. it is a comma-separated list of lane names). Also for any Task node you can also see a Lane property where you can insert the lane name it belings to. Also uploading your existing processes using lanes will work fine.
The visual representation should be done for the next version.