There is no option in Register.com's web interface to add a DS record for your domain name, however, their team will be able to add this record for you on your behalf.
You should send the formatted information below to [email protected] so they can help you get set up:
DS Record Key Tag: *YOUR_KEY_TAG* (i.e. 2371)
DS Record Algorithm: *ALGORITHM_NUMBER* (i.e. 13)
DS Record Digest Type: *DIGEST_TYPE* (i.e. 2)
DS Record Digest: *DIGEST* (i.e. 98DDA6DA4DEFBD26E74F201F2A98CBB976CA07889E86BAC0276A85A52B35C668)