You may need to log out of your Gmail account and then log back in after installing the HelloSign for Gmail plugin in order to start seeing the "sign" button on your attachments.
If that doesn't resolve the issue, we recommend uninstalling and reinstalling the plugin, then logging out of Gmail and closing the browser completely.
Next, reopen the browser and log back into Gmail again. You should see the "sign" button.
Note: If you are utilizing Google Chrome, it's possible for an extension to be installed but not enabled. Check to see if you've enabled the HelloSign extension.
On your Chrome browser, click the three horizontal lines on the top right hand corner of your browser> More Tools> then Extensions:
Once you are on the extensions page, scroll down and check to make sure that HelloSign for Gmail is enabled:
If you still aren't seeing the "sign" button, please send us an email at firstname.lastname@example.org and include the Gmail account you used to install the plugin, as well as the browser and version you're using, and we'll investigate.