you can't select it or it does not exist? if it does not exist in the
drop down you'd need to add the classification first before it becomes
available as an option in the list. go to admin > report suites > [pick
your report suite] > edit settings > and go to the traffic or conversion
variables and you should see a classification entry. click that and run
thru the set up for the variable you are trying to classify. once done,
return back to your issue and you should see it as an option.