Which product (Acrobat Pro/Standard, Reader), which version (including minor), which platform (Win/Mac), which OS/version?
I have acrobat pro 11.0.1 running on Windows 7
What is your workflow? Are you trying to do that from the "Sign" dialog or from the Digital IDs and Trusted Identities->Add ID dialog? If you get to the "Add Digital ID" dialog from the "Sign" dialog, do you have an unsigned signature field on which you click or do you just draw a rectangle for the signature to launch the "Sign" dialog? If you have an unsigned signature field, it is possible that the PDF's author associated with this signature field "seed values" that limit which certificates may sign this field. Do you have the same problem when you select Edit->Preferences->Signatures->Identities & Trusted Certificates->More->Digital IDs->Add Id? Do you have the same problem when you try to sign a blank PDF?
You have a very old version of Acrobat. You should upgrade it to the latest one. You are so far behind that Installer may not handle the update now directly to the latest version, so you may need to update in steps, to 11.0.03, them 11.0.05, then to 11.0.07 (there was a big change in the 11.0.07 installer, so do not skip this one), then to 11.0.10, and finally to the latest one, which is 11.0.12. An upgrade may fix many of your problems.