The user needs to belong to a user group with permission to access the Manager. That means creating a Context Access ACL entry for that user group.
I'd recommend *not* putting them in the Administrator group. For starters, you can create a new group and add the users to it with a Role of ContentEditor. See the Security tutorials at the site in my sig.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html