Seems to be a problem with the site http://cdimage.ubuntu.com/ . is there another way to download the the iso?
Is this what you need? Ubuntu MATE 16.04 (Xenial Xerus) Daily Build 
Ubuntu MATE 16.04 (04/2016) isn’t released yet, but there will be Alpha 1 at the end of the year. Like @mated pointed out seconds before me, there is a daily build, which seems to be operational.
Just in case you aren’t aware, Ubuntu (Mate) 16.04 is still in the development stage and should not be used by beginners!. 
If you already have 15.10 installed and like taking risks, use the following terminal command (Ctrl + Alt + t) and it will update your 15.10 to the dev version!. USE AT YOUR OWN RISK, ONCE INSTALLED THERE IS NO GOING BACK!.
sudo do-release-upgrade -d
See also:
As @wolfman stated, 16.04 is still under development and not meant for production machines. If you run Xenial (16.04) and if you encounter bugs, please report them.
You can either use the build in reporting tool (it comes up whenever an application crashes) or post them on Launchpad, maybe @wimpy can show us where to post them 