First, we are putting alot of time into adding designer documentation, so bear with us until this is done. In addition we are working on pretty editors for process and node properties - currently alot of them are text-based.
To answer your questions:
** process variable types **
Under Properties(BPMN Diagram), your process properties you will find a property called "Variable Definitions" where you can define process variables. Here you can define both the process variable names, as well as their type. For example if you enter there:
x:java.lang.String,y:java.lang.Object
this will produce the property definitions:
<bpmn2:property id="x" itemSubjectRef="_xItem"/>
<bpmn2:property id="y" itemSubjectRef="_yItem"/>
as well as the corresponding item definitions with the provided structureRef attribute values:
<bpmn2:itemDefinition id="_xItem" structureRef="java.lang.String"/>
<bpmn2:itemDefinition id="_yItem" structureRef="java.lang.Object"/>
So the sructure follows $varName:$varType and the whole list is comma-separated
** Condition Expression Language **
Yes, if you do not enter a value in this field, Designer will default to http://www.jboss.org/drools/rule. You can enter in values here which are either "drools" or "mvel". For script tasks and onEntry/onExit actions of tasks, the allowed values of the script language are drools and java, with the default value set to http://www.java.com/java if you do not enter in the value
Hope this helps.