You will need to also install the EPEL repository as nux-dextop depends on this for some of its packages.

Step 1: Install the nux-dextop repository

Run this command to install the nux-dextop repository.

yum -y install http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-5.el7.nux.noarch.rpm

Step 2: Install the adobe repository

Run this command to install the adobe repository.

yum -y install http://linuxdownload.adobe.com/linux/x86_64/adobe-release-x86_64-1.0-1.noarch.rpm

Step 3: Install the desired packages

This command will download the flash plugin for Firefox.

yum install flash-plugin

This command will download the java plugin for Firefox.

yum install icedtea-web

This command will install smplayer.

yum install smplayer ffmpeg

The following command installs a decoder and codecs which are required
to play certain types of media. Their usage may be legally restricted in
some areas. Obtain legal advice if you are uncertain about the laws in
your locality.

rpm -ivh http://mirror.centos.org/centos/7/os/x86_64/Packages/fribidi-0.19.4-6.el7.x86_64.rpm

yum install libdvdcss gstreamer{,1}-plugins-ugly gstreamer-plugins-bad-nonfree gstreamer1-plugins-bad-freeworld

Final note

Notice that you have the CentOS, Nux-Dextop and Adobe repos enabled by
default and there should be no issues with running "yum update" or "yum
upgrade" in the future.