Grey out Checkbox until selection made in ComboBox on a user form

I have a User form that people must select their name from the drop down list and then select a number of check boxes.
I wanted the Check boxes to remain disabled until a person actually selected their name from the combo box.
The person's preferences are sent to another sheet but some people are sending preferences without their name.
I was given a suggestion of 'CB01.Enabled = False', but could not get it to work.
The combo Box opens with a blank field initially, so that's when I would like the Check Boxes to be greyed out.
Could somebody tell me if there is some VBA code I can attach somewhere which will do what I want?
Thank you to anybody that can assist me.

It would help if you were a

It would help if you were a little more precise, does Excel have a drop down list?

On the form activate event

Private Sub UserForm_Activate()
Me.ListBox1.AddItem "a"
Me.ListBox1.AddItem "b"
Me.ListBox1.AddItem "c"
Me.ListBox1.AddItem "d"

Me.CheckBox1.Enabled = False
Me.CheckBox2.Enabled = False

End Sub

and on the listbox change event

Private Sub ListBox1_Change()
Me.CheckBox1.Enabled = True
Me.CheckBox2.Enabled = True

End Sub