An issue with the Gmail/Greasemonkey API in the latest versions of Gmail means that the Remember The Milk for Gmail Browser Add-on can’t currently load in Gmail. Unfortunately we rely on this API and the add-on can’t work without it. :(
We’ve reported the issue to Google, and they’re looking into it. In the meantime, if you’d like to access your Remember The Milk tasks in Gmail, you might be interested in trying the Remember The Milk Gadget for Gmail Labs, which doesn’t rely on this API.
We’ll update this post once the issue with Gmail is fixed, and the add-on is working again.