I see the problem. Apparently, the manual task doesn't have its own section at the moment, only the default section is included. You need to copy these classes:
1) Copy org.activiti.designer.property.PropertyBpmnFilter to org.activiti.designer.property.PropertyManualTaskFilter
2) Copy org.activiti.designer.property.PropertyBpmnSection to org.activiti.designer.property.PropertyManualTaskSection
In the new filter class, change
if (bo instanceof FlowElement) {
to
if (bo instanceof ManualTask) {
In the new section class, edit code at will to create controls for your fields, fill the values and persist to the model by, for instance, using the same mechanism we use with a FocusListener.
Now, you need to add an entry to the plugin.xml file. FInd the part where the extensions to this point are: org.eclipse.ui.views.properties.tabbed.propertySections. Copy one of those propertySection blocks and adjust them so you have this:
<propertySection
class="org.activiti.designer.property.PropertyManualTaskSection"
filter="org.activiti.designer.property.PropertyManualTaskFilter"
id="org.activiti.designer.propertyTab.manualtask"
tab="org.activiti.designer.propertyTab">
</propertySection>