There are a few different ways to install. First is a "bookmarklet". You create a new browser bookmark with the following code as the address, and click the bookmark anytime you're on a Numa map page to enable it. This method is the easiest to set up, but has the downside of needing to click the bookmark on each page. In many cases, this is the only option I know of.
Some browsers (read: Opera) let you set custom javascript for a site. Browser support like this will automatically enable the add-on for every map page you go to. If you're using Opera, save the following script to a folder on your computer. Then open an nmaps.net page, right-click, choose "Edit Site Preferences...", go to the Scripting tab, and select the folder the script is in. Using the add-on:
After you click the bookmarklet, or just load the page if you have a browser-based user script, scroll to the Comments section of the map page. Next to the word Comments in the header should be a new link, "Image Comments." Click that to open the image comments section and view all the current comments. If you're logged into Numa, you can add your own comment by simply dragging a box and typing your message.
In the near future, I'll add a page that lists maps with image comments on them. (With so many maps on Numa, who knows what maps will and won't have comments on them?) Maybe someday, I'll try to make it play nice with IE. And finally, if this gets popular enough, maybe you'll see it as part of Numa by default.
Edit: Here's a preliminary listing of commented maps. Might become more integrated, or at least more styled, soon. http://therealn.com/ic_maps.php