Hi,
I have looked through our resources here within Support Services, and found where this is a logged bug within our system. This issue has been brought to the attention of our ArcPad Development Team, and they are reviewing this logged bug. So, this is not anything you have been doing incorrectly, the option for this spatial reference system is currently not available within the Datum Configuration Tool, from within ArcPad. I was though able to locate a suggested workaround, which may be an option to assist you in your day-to-day workflow.
Thanks!
[#NIM094999 When using the Datum Configuration Tool in ArcPad 10.2, the NAD_1983_PA11 datum is not available as an option from the dropdown menu. ]
Suggested Workaround:
Take field data in the default coordinate system (usually WGS1984) and then bring the data back to your desktop where you can define it using the Project tool within the Projections and Transformations toolbox.
If you have any question, please feel free in contacting us, within Support Services.
http://support.esri.com/en/
Juan