It sounds like you are lacking the proper access rights to perform the action your are trying to perform. When users are first created they do not have access to do much.
Modify the users role.
Loosely copied from: http://resources.arcgis.com/en/help/main/10.1/index.html#/Modifying_a_user_in_Manager/0154000005rm000000/
- Open Manager and log in as the primary site administrator or a user with administrative access.
- Click Security > Users.
- Locate the user you want to modify and click the edit Edit icon corresponding to that user. This opens a dialog box that allows you to edit the user's password, e-mail, full name, and description.
- To change the user's role membership, click the add role Add Role icon to add roles from the Available roles list. Click the Delete icon Delete next to a role to remove it from the Member of list.
- Click Save to apply your changes
If you do not have the access to do this yourself then you will need to go to the machine you installed the server. Default install options require that some users are added to the system (not necessarily the user you are using)
Check active directory. There will be a GISAdmin user and and GISUser user and groups to match. If all else fails try to log in as the administrator of the server (named Administrator in a vanilla setup).
If you are not using Active Directoy the simplest way to lookup and access these users is run "mmc" (no quotes) from the run command line then add snap in -> Local Users and Groups. If the users are normal users they will not necessarily show up in the typical user dialog under the control panel (not documented, personal experience)
As a side note Windows Users can match names of users in the GIS interface, but they are separate entities. Administrator of a system is not necessarily administrator of a GIS system.