Find, FindNext, Replace and Go to Function in Notepad

Question

Sir, I am Doing a C# Notepad now.

I added   Menu just as windows Notepad using MenuStripControl.

My problem starts when I press menu item Find.

I created a new form frmFind to do this. this form has a Button(find), textBox(Searchtext).

 enabled isMDiContainer=true;

 I can't  access richtextBox Item by txtFind in frmFind.

 also want code for FindNext, goto, and Replace;

how can it possible, please reply.