As you can see the toString is showing the datatype
You are dealing with a result of Datatype: IEnumerable<Match> comparable to a particular collection of results from a RegEx action. Accessing the result you have to go into the collection and retrieve the different elements from it
yourMatchResultVar.first().ToString e.g. for first match
or iterate over it with a for each. Don not miss out to set the ArgumentType (for the for each activity settings) to System.Text.RegularExpressions.Match
thanks a lot I was getting the same error when I had first tried putting the Message box in a for each. But since I am only expecting the ID once this works great.