Setting a variable at run time

I am making an NSIS script which checks if a previous version of the application is installed. If so, it asks whether or not the configuration file should be imported from this previous version. So I have a global variable config file which I am trying to set at runtime depending on whether the user chooses "yes" or "no". The problem I am having is when compiling it complains with File: "${XMLConfigDir}*.xml" -> no files found. because XMLConfigDir has not been set yet. So is there anyway to set a variable at runtime?