"How do I attach the signed form?"
Unfortunately, the latest update has completely removed the attachment functionality (it is missing entirely).
This issue has been reported and you will have to wait for the developers to implement a fix.
You can always file by mail which is by no means ideal, of course, but it is an option.
Thank you for letting me know that it's just not my ignorance that's keeping me from getting the signed form 8453pe attached! I did it last year with no problem, and couldn't figure out what was wrong this year. Any idea when or how/if we will be notified when developers have fixed this ridiculous bug?
I do not have any inside information but, if past is prologue, there probably won't be a fix until next week. I am hoping it will be early next week given the gravity of this bug, but they usually release updates toward the end of the week,