Did you refresh the form?

Not sure how much into the  beginners lessons you are at.   I just looked and there is plenty of material in the tech help.  I just don't know what level I should be speaking to you at as the refresh is a VBA topic.   Tell me how far into the lessons you are so I can try to walk you through this.  

Try these Value From a Form and This

yes, I'm a beginner here but I've been working with access for a while via youtube. Followed various DIY movies. So I know a little more than beginner level.

I have a form where I register a supplier for my jewelry. But if that supplier is not in the combobox list, I want to be able to click a button and a new form opens where I can create this supplier. Then when I close that form, the new supplier should appear in the combo box for suppliers.

So if your combo box is bound to a table, just add a button that opens your form for suppliers,
DoCmd.OpenForm "formname", , , , acFormAdd
then make an on-close event in the suppliers form
Forms!FirstFormName.Requery
and then it should have your name in the combobox.

Suppliers hava a table for them self. I dont get it to work??

So you have a form that you use to look at or add suppliers with right? (If not how are you adding suppliers now?) Put that form name in formname inside the buttons onclick event that we are making
DoCmd.OpenForm "formname", , , , acFormAdd

Once inside that form (the form for suppliers) we add a on-close event to that form that sets the original form to requery.

This works, I do it all over. What do you mean by I dont get it to work? Do you get an error or anything? What is the code you're putting in look like after you replace my temporary values with your real values?

I would Requery the ComboBox.

Forms!FormName!ComboBoxName.Requery

Watch the Cascading video.  Starting about 13 minutes in Richard shows you how to refresh the information for the combo box.
I know you are not doing cities and states but it is the same concept.

I Do what you say but another supplier gets active.