The single bay My Cloud units DO NOT officially support third party apps like the more expensive My Cloud units do.
Unofficially there might be ways to install Joomla but to do so would require using SSH to modify or replace the My Cloud OS/firmware. One such way to run web hosting for v4.x My Cloud’s is explained in this thread:
http://community.wd.com/t/app-webhosting-for-firmware-v4-10-2015/94892
