Incidentally, I'd wrap the dlookup in a Nz function - Nz(dlookup(....)) or Nz(dlookup(....),0) just in case MemberID is Null. Also, you'll probably need to move the quotes so they don't include MemberID, thus:
Nz(DLookUp("[LastName]","tblMembers","[tblMembers].[MemberID]=" &...