Hmmm, I answered a similar question a couple of days ago? Is this a double post?
Here is the link to that 'discussion', where I posted an answer, which I'll repeat here:
Slide numbering - eLearning
My answer:
I would define a user variable v_slide. Insert it in a text container on the Main Master slide: the Title master slide in any Captivate theme doesn't use objects from the Main Master slide. So you'll have that variable only on all the other slides, based on any content slide using objects from the Main Master slide. You may have to check that, if you also used the Blank master slides which is not taking objects from the main master slide neither.
You have to use the Expression command to calculate the correct value of v_slide because that command is not available as simple action (in the Actions tab). Create an advanced action EnterSlide with two commands:
Expression v_slide = cpInfoCurrentSlide - 1
Continue
Then select all the slides in the Filmstrip, (Title slide may be included) with CTRL-A and use the On Enter event in the Actions tab to assign that advanced action EnterSlide.