You have here everything you need to get it working.
yum install yum-utils
rpm --import "http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF"
yum-config-manager --add-repo http://download.mono-project.com/repo/centos/
Run a package upgrade to upgrade existing packages to the latest available. Then install Mono as described in the Usage section.
The package mono-devel should be installed to compile code.
The package mono-complete should be installed to install everything - this should cover most cases of “assembly not found” errors.
The package referenceassemblies-pcl should be installed for PCL compilation support - this will resolve most cases of “Framework not installed: .NETPortable” errors during software compilation.
The package ca-certificates-mono should be installed to get SSL certificates for HTTPS connections. Install this package if you run into trouble making HTTPS connections.
The module mono-xsp4 should be installed for running ASP.NET applications.