The registration number for an ArcPad license is found in the Information drop-down, by clicking the question mark icon on the ArcPad menu > About ArcPad.
If this section does not exist, or if the product is not yet registered, log in to MyEsri, Click the My Organizations tab. On the Licensing tab, click View Unlock Codes.
If this section does not exist in My Esri, look for the original email from Esri Customer Service with the registration codes. If this cannot be found, contact Esri Customer Service.