When inserting a model that has pre-defined flexibility the Varied Items Table under parameters shows the Orig Value and inserts this into the New Value, there is no drop down menu to show the names of the other values.
The only way to find out the parameter names is to open the sub-assembly, select and edit the definition, then check it out of Windchill, select the placement tab to view the parameter names, make a note of these as this information will not be available when the you change the flexibility, cancel the edit definition, undo checkout, open the top level assembly window, select the sub-assembly, select and edit the definition, select the Flexibility tab, then the Varied Items button then the Parameters tab, then type in the required New Value.
This currently takes approximately 20 clicks and some typing instead of 2 clicks.
Hopefully this will be made available in Creo 4.0
If I understand your request correctly, the drop down list is available in Creo 3.0 M090.