You need to have a reference to that form in your main form.  Then when you
select the menu option to show that form, you can create a new instance and
assign it to that variable.  Then call Show().  You should also create a few
events in that form so you can handle the button clicks in the main form.  
Attach all of these before you call Show().  Here is some sample code of what
I mean:

This goes in the FindReplaceForm:

       public event EventHandler<FindEventArgs> Find;

       protected virtual void OnFind(FindEventArgs e)
       {
           if ( this.Find != null )
           {
               this.Find(this, e);
           }
       }

Then create the FindEventArgs class:

   public sealed class FindEventArgs : EventArgs
   {
       private readonly string find;
       private readonly string replace;

       public FindEventArgs(string find, string replace)
       {
           this.find = find;
           this.replace = replace;
       }

       public string Find
       {
           get
           {
               return this.find;
           }
       }

       public string Replace
       {
           get
           {
               return this.replace;
           }
       }
   }

In the button click handlers on the FindReplace form you will create an
instance of the FindEventArgs class, and then call OnFind() to trigger the
event.

Your event handlers in the main form will be able to use the FindEventArgs
instance and get the information you need.  You might need more than just one
event and the EventArgs might change depending on what you want to do.  This
just gives you a start.