I'm afraid that you can not create automatic (smart) collections in the iPad version.
As you've discovered, you can create collections and add individual files to them using the double square icon top right.
You can also add multiple records.
First you have to create the collection as before. Then select the Library button (top left). There will be a collection button to the right of library list. It's grey and shows the number of collections in the library. Tap on it and select the collection you want to add too.
At the bottom left you will see an icon with lines and a plus sign. Tap on it. This will give a a list of all the records in the database with a radio button to the right of each item. Every record you select will be added to the current category. Notice that you have search bar at the top. You can narrow the number of records to scroll through by using the search. The sort icon at the top left will let you change what is shown in the list and it's sort order.
If you have a Mac, you can get the OS X version of Bento. In it you can create smart collections. These collections will sync with the iPad version and continue to add new records based on the criteria set up on the Mac.
This, among many other features, is why I normally don't recommend Bento on the iPad unless you also have a Mac and will get the full version to sync with. It's missing too many features as a stand alone.