I found the following for ACS 5.8
Hopefully it is the same in 5.5
http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_secure_access_control_system/5-8/ACS-ADIntegration/guide/Active_Directory_Integration_in_ACS_5-8.html
Supported Username Formats
The following are the supported username types:
■SAM, for example: jdoe
■NetBIOS prefixed SAM, for example: ACME\jdoe
■UPN, for example: jdoe@acme.com
■Alt UPN, for example: john.doe@acme.co.uk
■Subtree, for example: johndoe@finance.acme.com
■SAM machine, for example: laptop$
■NetBIOS prefixed machine, for example: ACME\laptop$
■FQDN DNS machine, for example: host/laptop.acme.com
■Hostname only machine, for example: host/laptop
Should be good to go
I'll be using NetBIOS prefixed SAM, for example: ACME\jdoe