The snmp-mibs/mdraid-snmp-mib package is not currently available as part of the official Gentoo Linux distribution but is distributed using an overlay maintained and supported by us at Hacking Networked Solutions. Before the snmp-mibs/mdraid-snmp-mib package can be installed therefore the overlay must first be installed by following the instructions in the Installing the Hacking Networked Solutions overlay for Gentoo Linux section of the Hacking Networked Solutions overlay for Gentoo Linux documentation.
Once the required overlay has been installed the installation of the snmp-mibs/mdraid-snmp-mib package is as easy as any other on Gentoo Linux and is performed using the emerge application as usual as shown below.
Once you are happy that the snmp-mibs/mdraid-snmp-mib package will not install any dependencies you do not require, and that any dependencies which will be installed have the correct use-flags set, you can install the package as shown below.