Officially, no it is not possible. Neither the first gen v4.x nor the second gen v2.x single bay My Cloud units officially support adding third party modules like Plex. This has been asked and discussed in a number of prior discussions that can be found using the forum search, magnifying glass icon upper right.
Unofficially it might be possible to install Plex to the first gen v4.x firmware single bay My Cloud but it appears to require using SSH to possibly replace the existing My Cloud firmware/OS with Clean Debian. See the following post/discussion for more information:
http://community.wd.com/t/using-plex-media-server-with-my-cloud/94429/12
