You need to change the class in the script editor called Scene_Menu
Here's a little tutorial on what you need to do.
1. Find the s1= s2= s3= etc near the top of the script. This is the initialization of the text that will be displayed in the menu. For example if you put s7="Materia" then "Materia" will be displayed in the menu.
2. Now you need to add the instance variable that you created to the menu. For example if you created s7="Materia" then add s7 to the list in the Command Window. It should read somthing like command_window=Window_Command(160, [s1,s2,s3,etc]). Just add your s7 to the array of s stuff. It should be located directly under step 1.
3. Now you need to call the script from the menu. Just scroll down until you see a case call and a bunch of when 0 blahblahblah when 1 blahblahblah when 2 blahblahblah. You need to create a new one. It is a little wierd but
s1 corresponds to the when 0 statement
s2 corresponds to the when 1 statement
s3 corresponds to the when 2 statement etc
So if you want to add a 7th item to the menu then you have to create a when 6 statment. So just type
when 7
$scene = Scene_MateriaEquip.new
end
It's that simple. You can also customize it by changing the color or the selectability, but just look at the other when statments and you should get the idea.