Well, I think what you may not be understanding is current(). Where is your calculated value located? Is it in a section bound to the same field that is bound to the dropdown? I'm thinking probably not... so I'd expect something perhaps more like:
xdXDocument:GetDOM("GetFormsList")/dfs:myFields/dfs:dataFields/tns:GetFormsListResponse/tns:GetFormsListResult/tns:Item/tns:DisplayName[../tns:Value = current()/my:FieldThatIsBoundToTheDropDown]
If you need to figure out where you are, you can always try just the local-name function in your calculated value - that will tell you, so you will then know what current() means in your particular instance. Does that make sense?
(PS - I did a webinar on this recently - you can find a recording here.)