I had this problem myself and really don't understand why this is so hard to find. My wife used a year ago and I wasn't able to find this feature again, so I looked in her browser history and found it. There isn't even a short accessible link, I can give you only this. Fill in your keywords, set the radius and in the lower left you can activate the alert. You need to be logged in with a gmail though to use the alerts.

Also it may not work on mobile and sometime I have problems with Firefox as well. If you want to be sure use Google Chrome on a desktop for this. I wouldn't be surprised if this is another stab to Firefox users.