Q:

How to add a string to a listbox in another opened form?

I am attempting to add a string from frmAddSegment to the listbox in frmMain. frmMain is open when doing this process, just for some reason the string is not being added to the listbox.
I've checked the string I'm attempting to add the listbox by using a message box and it is working fine, it's just not getting to the lsitbox. The modifier property on the listbox has also been set to public
frmMain fmain = new frmMain();
    fmain.lstbxSegments.Items.Add(segmentPBMin.ToString()+":"+segmentPBMin.ToString()+"."+segmentPBMils);

I expected the listbox to contain a new item however it remains blank.

A:

frmMain fmain = new frmMain();

You are creating a brand new instance of "frmMain", and adding your item to that instance rather than the one currently running. You should instead do:
the_Form_That_Is_Open_Right_Now.lstbxSegments.Items.Add(... your code here);

If you're having trouble finding where your form is created, you can hit Ctrl+F, make sure the filter is set to "Entire Solution", and search for new frmMain(). You might see something that looks like this:
Application.Run(new frmMain());

You can store that instance in a variable like so:
frmMain yourForm = new frmMain(); 
Application.Run(yourForm);

