When you click on Pimwick Plugins -> PW Gift Cards you should see a “Create Check Balance Page” button. This button will automatically create the Page for you with the appropriate Shortcode.
If you do not see this button that means the Page already exists with the Shortcode attached. Click on the Pages menu in the left to locate it.
For reference, here is the shortcode:
[pw_gift_cards_balance]
Hope this helps, let me know if you have any other questions!