Here is an example usage of the runScript (as seen in the Selenium IDE source tab):
Second: I think you need to be able to access the variable you want to update/set.
This can be hard if the IDE cannot acces its scope directly. Maybe you or the developers can add a Global function to the page so you can call it with "functionName();" also see https://stackoverflow.com/questions/9354191/calling-custom-functions-from-firebug-console for more idea's on calling functions from the browser console (or the runScript command, which is comparable)
You should be able to practise/test run the commands from the Firefox console before you add them to your Selenium scripts, since this can be hard to debug from the IDE.