Whether you use the OTGS Installer plugin to add Toolset plugins to your site or you have downloaded plugins and uploaded them manually to your site, you need to register Toolset via the custom installer page, which you can arrive at by clicking one of the register links from the plugins page (see screenshot).
Then, when clicking the Register link, you'll see a link to "Get a key for this site", which will take you to the accounts page of this site and generate a key for your site, that you can then paste into the field back on your site.