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
And then… register your ActiveInbox Account (if you have it!)
Click 'Connect...' in the topbar:
Then fill in your ActiveInbox password: