Hello,
There are two ways you can open the file for editing in the WYSIWYG editor.
SNIPPET (only the code between your snippet tags will be editable)
WHOLE FILE (the entire file will be edited)
There are four different User types in Snippetmaster. You can read in the documentation about how they work here:
http://www.snippetmaster.com/documentation/admin-users.html
If you want your customer to only be able to edit the SNIPPETs you have created for them, then I do not recommend giving them the Admin user privilege. Otherwise, they will be able to edit the WHOLE FILE, and will probably screw up the webpage.
I recommend givng them either Power User or Regular User privilege, which will allow them to only edit the SNIPPETs you have created for them. This way, they can only edit the specific areas of the webpage you have specified as editable.
I hope that helps to clarify how things work. Please be sure to read the documentation to undersand how the User privileges work, and here is some documentation on how to add SNIPPETs to your webpage:
http://www.snippetmaster.com/documentation/snippet-tags.html
** Note: If the user logs in with Super User or Regular User privileges and the file is not listed, then this means you have not added any Snippets to the file, or you did not add them correctly. The file will only be listed as editable with SNIPPETS if there are valid snippets found in the file. If this is the situation, then please make sure you have properly added opening and closing Snippet Tags.
Let me know how things go.