I assume that when you say "K-form" you mean a Schedule K-1. There is no specific single form for adding Schedule K-1 information to your tax return. That's why you can't find a form, and your tax preparer can't give you a form. The Schedule K-1 information doesn't all go on one form. The various items from the Schedule K-1 have to be entered in different places on your tax return. It's best to use software to do this, or have your professional tax preparer do it.
And you can't just prepare an "addendum" to your tax return. You have to file an amended return on a special form, Form 1040-X, which involves recalculating your entire tax return. Again, it's best to use software for that, or have your professional tax preparer do it.
You said "My taxes were quite simple."
"Were" is the right word. Your taxes are no longer simple.
