Rafael,
I had the same issue. You need to go into ACS and create a custom Shell Profile (Policy Elements > Authorization and Permissions > Device Administration > Shell Profiles), flip to the "Custom Attributes" tab, and add the following:
Attribute: cisco-av-pair
Requirement: Mandatory
Attribute Value: Static
Value: shell:roles="network-admin"
...although if you want a non-admin or DCNM "User" role, you would use the following instead:
Value: shell:roles="network-operator"
Save that. Then make sure your Device Admin Authorization Policy (Access Policies > Access Services > Default Device Admin > Authorization) references that Shell Profile in the "Results" section.
I'm using DCNM version 6.2(5) and this works.
Here's a useful link for more info: http://www.cisco.com/en/US/products/ps9911/products_configuration_example09186a0080bf5512.shtml