calling a method from another form

First of all, the populate() method needs either public or internal access to be called from another class in the same application.

If it's a static method, you can simply do:

Form1.populate();

If it's an instance method, you need a reference to the Form1 instance. 

You can get a reference like this:

Form1 f1 = Application.OpenForms["Form1"] as Form1;
if (f1 != null) f1.populate();