Here's a step by step guide that helps you add a form to your main menu:

In Joomla 1.0.x

1. Go to Manage forms, and click on Preview next to the form you want to add
2. Copy the link (where you are taken to)
3. Go back to Administrator > Menu > mainmenu > New > Link-url
4. Give the new menu item a title, and paste the previously copied code

In Joomla 1.5.x

Solution 1: 

1. Go to Manage forms, and look at the last column (form id). Let's assume your form id is 1
2. Go to Administrator > Menu > mainmenu > New > Internal > RSform!
3. Give your menu item a name, and on the right side, in the Parameters tab, in the Form Id textbox type 1 (the form id)

Solution 2:

1. Go to Manage forms, and click on Preview next to the form you want to add
2. Copy the link starting from index.php(something like: index.php?option=com_forme&fid=1)
3. Go back to Administrator > Menu >mainmenu > New > External >
4. Paste the link and give your menu item a title.