If you can't see the free Aspect Finder (aka Item Specifics) tool in Optiseller, then follow these instructions:
Click on "Tools & Services" from the menu on left hand side
Then click eBay Aspect Finder
Click on Subscribe
Then you can either select your store or add your store details and follow the instructions to subscribe
Once store is selected and linked, click complete.
You should then select the relevant category
Then click "Start" (at the bottom)
Your report will take a short period to be compiled. Once ready, Optiseller will email you to let you know.
While it is running, you cannot run it again until it completes: