You need to tell the Publisher where to get the Certificate from:
- Go to Project-> Properties
- Go to the tab named “Signing”
- Check the Checkbox labeled ‘Sign the ClickOnce manifests’. This enables the section where you can point to a certificate.
- Click on the button labeled ‘Select From File…”. In the directory listing, choose a key file (.pfx) and Open.
- From the step 3, click the button labeled “More Details…”. This will display the certificate you’ve just selected.
- Click “Install Certificate” and follow the prompts.
- Save and Publish your Solution.