IW5M is a modification of Modern Warfare 3 aimed to "offer a proper online-enabled experience for IW5, featuring well-familiar features such as friends lists, server-side statistics, semi-limited modification support, the upcoming TNet service and downloadable content support."
It was created by (nearly) the same people who brought your alterIW. Note that matchmaking is not included like IW4M otherwise it'd be emulating Demonware's servers and will result from another C&D from Activision (which is how alterIW got closed).
It was in a closed beta for a while but now it is available for public to download. You will need a copy of MW3 in order to run it (although you don't have to purchase the game in order to play). Links for this will be available soon, just download it yourself for the time being.
Get the DLC files here: iw5m (make note of the little message under the cupcake/muffin).
Get the client files here: https://iw5.prod.fourdeltaone.net/files/iw5m-client.zip
Place the DLC files in the correct place according to the paste and put the client files in the game directory and run LaunchIW5M.exe. If you run into any issues or problems, I recommend that you ask on the FourDeltaOne forums where a dedicated team of Tech Support staff can help you. Make sure to check here before asking (especially if your MW3 is not English: https://fourdeltaone.net/viewtopic.php?f=30&t=9515)
Like I said before, matchmaking is NOT supported. You need to join servers via the Servers button. Go to Options > Dedicated Server > Enable Server Browser in order to list servers.
For a list of current servers and server status visit here: FourDeltaOne/SecretSchemes Server Status
To set up your own dedicated server, read this: https://fourdeltaone.net/viewtopic.php?f=30&t=9927
IW5M is running on 1.4.382 as many speculated but hacking is to be done at your own risk, manual bans can be applied for level hacking.
Have fun.