If you recently upgraded to a newer major version of Foxit PDF Editor and you do not have your Digital ID Certificates under Protect> Sign & Certify> Digital IDs:
The digital certificate IDs will have to be created from the existing .pfx or .p12 file that the original Digital Certificate IDs were created under. Select Add ID in the top left corner> My existing digital ID from a file>navigate to the digital ID file and input the password.
You can then choose to name it and input the email address you want associated with it along with any additional organizational info. You should then see it appear under the list of digital IDs
You can then navigate to Protect> Sign & Certify> Place a Signature
Draw the field for the signature on the PDF and the following window will open. You can select your digital ID from the dropdown for signing: