All existing signatures are displayed as tiles on the Home page.
If you want to enable an email signature that has been previously disabled, hover your cursor over the required signature tile and use the Enable option:
This displays the Applies to page. This page varies according to the email service that you've configured for Exclaimer Cloud - but essentially, you can enable/disable options for server-side and/or client-side signature processing. For example (options shown for Office 365):
From here, you should 'switch on' the Apply this signature option, at the top of the server-side tab (if required), or the client-side tab (if required). Switching on at least one of these options, will enable the signature.