Using Strings in input or output connectors is not very common in Modelica. There are currently no standard connectors for Strings available in the MSL. Nevertheless, the SerialPackager package provides blocks for Strings, too. The use of this blocks is demonstrated in this example.
Please note: The model should work with Dymola 2014, but seems not with Dymola 2013 FD01 (and probably previous versions).