The default admin name is admin, but it can be changed. "Client" though, suggests that the original programmer is still the admin Super User. I'm rusty at MODX Evolution, so I can't give you detailed instructions, but see if you can get to where you edit roles and see if the "Client" user has all rights.
I've never heard of the "SEO Override" menu. It might be from an add-on component or some custom programming by the original programmer.
If there are no meta tags in the source code, you have the the opportunity to do it right and forget about the input boxes. The best method is to create a Template Variable for each type of meta tag you want.
For example, create a Template Variable called "meta_description" and attach it to your your main template and any other templates you want to show the meta description tag. Then put this tag in your template where you want that meta tag to appear:
Normally, that would look like this:
<meta name="description" content="[*meta_description*]" />
The tag will be replaced by whatever you put in the TV for each page.
There's a great tutorial and example here:
http://sottwell.com/how-template-variables-work.html.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html
