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Hi,
when you install a module Its adds a link to the Main menu, you can define the module access rights in the Administration Menu - system- groups in which you can give rights to Users who can see and access the module as per your preference,
like the group Registered User, Anonymous Users.
In the System -> modules administration you can set the Oder like 0= hide and 1,2,3...X to each module as per your choice, if you set 1 for NEWS it will be on top followed by the oder number.
To display a custom HTML page in your main menu you can use any of this module Mypage, TinyContent or TinyD.
else check the FAQ..
https://xoops.org/modules/smartfaq/faq.php?faqid=280this will help you to creat a page "about" page.
Regards,
Argha