how to add the userforms to combobox and open it when pressing the command button ?

I assume you mean a activex button on the sheet?

Developers Tab -> Design Mode -> View Code.

At top of window, select Commandbutton1. IDE should insert the Click event. Enter the name of the macro you want to run in the click event.

E.g.
Private Sub CommandButton1_Click()
RunMyMacro
End Sub

You can also double click the control for a shortcut. Just make sure you're in design mode. And don't forget to turn Design mode back off.

Good luck!