First, install into Chrome
Click Install to get started
A confirmation question should appear at the bottom of Chrome, click 'Continue':
After that, another box will popup asking for confirmation. Approve it.
Troubleshooting: if it doesn't appear in Gmail, and isn't in your Extensions list, try installing directly from Chrome's Webstore.
First, confirm you wish to install into Firefox
You may see a security bar at the top of Firefox, click 'Allow' to continue:
A confirmation box will appear. Click 'Install Now' to accept.
(If you don't see any of this, please click Install to start the install manually)
Download ActiveInbox into Safari
Next, check it appears in Gmail
After it has installed into your browser, refresh Gmail (or Google Apps Mail).
If it is successful, you will see the new Review Bar at the top of Gmail.
And then… register your ActiveInbox Account (if you have it!)
Click 'Register...' in the topbar, then 'Register ActiveInbox Plus to this computer' and enter your ActiveInbox Acccount password.