I'm assuming you have put a combobox directly on a slide?

Right Click > View Code

You should see two lines of code and above two dropdown menus that (probably) say ComboBox1 and Change. Alter the Change one to "MouseDown"

Between the NEW lines of code paste this:

[VBA]With Me.ComboBox1
.Clear
.AddItem "Mastercard"
.AddItem "Maestro"
.AddItem "Visa"
.AddItem "American Express"
End With[/VBA]

Now you just need to do whatever based on the text in the box

I would suggest a command button with code like

If Me.ComboBox1.Text="MasterCard" Then .....