The primitive datatypes such as integer or string can be added in the outline of a process, but they can not be selected in the input or output dialog box - only the data is visible there.
Rethink the meta-model concerning these elements (only data parameter selectable, PrimitiveTypes are simply PackageableElements and therefore not useful for applications right now) and make changes to the code.
See the metamodel-discussion on http://following-flo.blogspot.com/.