Adding an Text Box entry to a Combo Box

I know, it seems pretty simply, but its really annoying me! hahaha

I am attempting to make a program that will hold stock items in a list view. When adding a new item, an option for adding a category via a combo box is given.
On a separate form, users can add a category to this combobox.
I have included images below to clear this up a bit,

Form 1 (The form that shows the items, not really necessary)

Form 2 (The form where items are added)

Form 3 (Where users add a item to the combo box)


I've tried so many different ways of doing this, but really dont know what to do. I'm thinking about saving the contents of the dropbox to a textfile, as thats the only way I really know how to do it. But other than that, I really dont know. Nothing works WELL.
Sometimes I can get it to add items, but then it doubles all the items, and other times it only prints one item and the rest are black entries.

Anyone with any help is more than appreciated
Thank you guys!!

Attached Images