Here is a methodical list of steps that you can follow if you have tried a few things and can't get Sortd to work:
1. Ensure no other Chrome Extensions are conflicting with Sortd
There are two ways that you can rule out possible conflicts with other Gmail extensions. The first is to disable any other Gmail extensions you are running (even if only as a temporary measure). You can do that by going to chrome://extensions and disabling your Gmail extensions one by one.
The second option is to install Sortd on a new Chrome Profile - if you prefer to try this please see point 6.
2. Check that the Sortd Chrome Extension is enabled
Go to the Chrome Menu > More Tools > Extensions, then scroll down to Sortd. If the extension is disabled, check the box to enable it and then refresh Gmail. If the Chrome Extension is working you should see a Sortd tab on the far right hand side of the screen.
3. Enable 3rd party cookies
Sortd needs 3rd party cookies to be enabled for *.sortd.com, please either enable 3rd Party Cookies in Google Chrome or add an exception to allow 3rd party cookies for *.sortd.com. Click here for detailed instructions.
4. Ensure that Sortd is able to access your folders via IMAP
Sortd requires IMAP access to be able to access metadata about your email (See Does Sortd store my email) as well as access to the Google+ API's.If you are using a Google Apps domain account it is possible for your Google Apps Domain Administrator to deny access to IMAP and the Google+ API's in which case Sortd will unfortunately not work. Note that you don't actually need a profile in Google+, just access to the Google+ API's. This is because Google now do all authentication through Google+ and not simply through Gmail anymore.
5. Reinstall the Chrome Extension
6. Install Sortd on a new Chrome Profile
There's a chance that Sortd is clashing with other Gmail extensions you have installed.
To install Sortd on a new profile - in Chrome go to the menu “People->Add Person” and create a new ”test” chrome profile. Alternatively if Menus are not enabled look for the avatar (normally in the Chrome title bar) which will allow you to create/switch profiles.You will then be given a new Chrome browser screen and asked if you want to login. You don’t need to do this if you don’t want to. In the new Chrome browser tab go to your normal gmail and login.
Once you have done that you will need to install Sortd in this new Chrome profile by going to www.sortd.com/install and downloading Sortd again. This should then open a new tab with your email in it along with Sortd (you can close the first tab you opened). You can then try and login/activate Sortd.
If Sortd wasn't working on your primary profile and it does work in this new profile - something has possibly become corrupt with your cookies or data in your main/normal profile (in which case you should delete your cookies), or another Chrome Extension or Plugin is preventing Sortd from working correctly.