So I wanted to add an option to choose their avian companion to the Master Falconer subclass I created; so far so good.  Added the class feature, typed out the description,  said it had options, added a creature to the feature, chose rule type choose creatures, creature group beast companion, added the birds I wanted the character to be able to choose from; so far so good ... until I tried to make a sample character to test it out.  In the character builder, it gives me the class feature, with one choice associated with it, but instead of giving me a list of the birds I put in the feature, the dropdown menu where you should be able to choose the bird just says "Choose a level 3 option", and that's the ONLY thing it says.  I thought maybe the problem was saying that the feature had options, so I unclicked that box and kept the rest of it the same, and now it doesn't give me any menu to choose a bird at all.  How do I make this work?

I just wrestled with this last night.  You need both the option for options checked off like you had originally and you need a comma delimited list in the box to the .... right of that which will indicate at what levels you can choose from.

Then you need to make a new CLASS FEATURE OPTIONS for each creature you want to make selectable, and set the level you want that option to be available at (which can include a creature I think).  The options drop down will pull from these options if the character meets the level requirement.

You may also need to reload the character editor for the options to show up. I had an instance where none of my changes were showing up without a full refresh (including de-selecting the subclass and re selecting it).

Thanks so much!  Worked like a charm.