Ok,

So you have a stock osCommerce ms2 template. You have the header.php and footer.php files and the index.php file in ms2. These make up the site structure. You also have a stlyesheet.css file which handles background graphics and fonts and colors.

In osCMax, all you need to do is make a new template folder. I will call it custom. You make this folder in /catalog/templates

Then, copy all the files from the fallback template folders to the custom folder.

Next, you need to modify the main_page.tpl.php file in the /custom folder by adding the html from the header.php, index.php and footer.php files from your ms2 php files.

This html should replace the existing html in this file. You need to use your skills in html to do this correctly, but it is straight html, so it is not that hard.

Then you need to replace the styesheet.css file with the one from your template.

That should do it.