I can't get the + to show. Under Custom in the number formatting options,
what exactly do i type in the space to get this to work?

I tried
+0.0;-0.0
for the custom formatting type and it seems to display what you want.

Notice that for zero it shows
+0.0
If you don't want the + to show in this case, try this instead
+0.0;-0.0;0.0