You'll usually see the "No filters available" message if your Event field is still set to All Events.

If you change this to a specific event, you'll then see additional options if they are available for your account and you'll be able to use the Filter by field as expected.

Currently, the Filter by field allows one selection for event filters.

If you need additional filtering, we suggest creating a Custom Event in your Mixpanel account, including any filters needed there. Then you can display the Custom Event in Geckoboard.