First of all, please make sure that you have Freemake Subtitles Pack key. Please remember that you can't activate Freemake Subtitles Pack with Gold Pack key.
1. Copy the key from the email.
2. Open Freemake Video Converter.
3. Go to Help-> Get Subtitles Pack-> Click “I already have a key”->Insert your key in a field. Make sure you copied only the key without additional spaces. Once you delete the space, the OK button should be enabled.->Click "OK". See the message that you've activated Subtitles Pack. That’s it. You can enjoy Subtitles Pack features now.