How do I post this in a post?
Do you want the $MFORM_3$ to be in your one of your threads or in index page and call it something like "Submit"?
I will assume you want to create index page and call it "Submit"
To create index page first you need to log in to your website as an Admin
You should see now an Admin bar on top of your website
Click on Addition and than on the Page Editor
A new window should open
For the Page name: call it New Submit or what ever you want to call it and for the Page content: paste in your $MFORM_3$
Important don't start the page name with Submit call it something else like New Submit and later you can rename it into Submit if you want. Why? Lets just say it's the uCoz thing, you will get page not found error.
Don't forget to Save it and your done!
Now you should see a Submit page in your Main Menu
Click it and see how it works, if you will encounter problems or page not found let me now.