I need to add my Digital Signature with a CAC as my defaul and it wont let me

When I follow this steps, I can't get the last one done as it specifys on the guide. It sends me to selec a file. Where do I choose the digital ID?

Specify the default digital ID

To avoid being prompted to select a digital ID each time your sign or certify a PDF, you can select a default digital ID.

Choose Edit > Preferences > Signatures. In Identities & Trusted Certificates, and click More.
Click Digital IDs on the left, and then select the digital ID you want to use as the default.
Click the Usage Options button , and choose a task for which you want the digital ID as the default. To specify the digital ID as the default for two tasks, click the Usage Options button again and select a second option.

A check mark appears next to selected options. If you select only the signing option, the Sign icon appears next to the digital ID. If you select only the encryption option, the Lock icon appears. If you select only the certifying option, or if you select the signing and certifying options, the Blue Ribbon icon appears.
Note:To clear a default digital ID, repeat these steps, and deselect the usage options you selected.