Library Folder Structure

The extracted library must be in the following directory before enabling the module: sites/all/libraries/signature-pad/build

The "/build/" wasn't obvious in the documentation, and not how the signature-pad library is packaged. I also didn't see a way to correct this without uninstalling and reinstalling the module after fixing the path issue.

If you are missing the "/build/" directory, the first time you enable the module you will get the following error:
"Warning: file_get_contents(sites/all/libraries/signature-pad/build/json2.min.js): failed to open stream: No such file or directory in drupal_build_js_cache() " - but then you won't get any other errors, the field simply won't work.