class "MxModel" does not extend that class

we are very new to R and OpenMx and are trying to get it running on Debian GNU/Linux Lenny. We downloaded and (successfully) installed the openmx-package. But when we try to run our Script we get the following error:

##
Error in initialize(value, ...) :
cannot use object of class "character" in new(): class "MxModel" does not extend that class
Calls: DAF_simulation ... mxModel -> firstArgument -> new -> initialize -> initialize
Execution halted
##

On Windows the same script with the same OpenMx-Version runs just fine and this is what we don't understand, because it seems like a structural error in the MxModel class.

Our R version is (due to Debian lenny) somewhat old : R version 2.7.1 (2008-06-23)
Do you know what might be the problem here?

regards,
Michael