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).