First, you should really not be creating a new site using Weaver II pro which is now totally obsolete and unsupported.
You should use the new Generation Weaver Xtreme (free) instead. It uses the same logic as Weaver II pro, so you wont be lost, but it has many more features and solid code base.
There is a conversion procedure if you are far along the design
That said, if you create your menu using WordPress Custom menu in Appearance > menu (as you should), you can add any Link as a menu item and put an outside link in there.
You can look for WordPress Custom menus Tutorials on Youtube if you need any detail exploration of what they can do