To access MK3.Org Grid you will need a virtual worlds viewer. Below are recommended viewers.

Firestorm Viewer

You can download a copy of the free Firestorm OS viewer from the official Firestorm site HERE which is a fully featured viewer and by far the most popular of all the viewers that are out there.

Adding the MK3 Grid

The MK3 Grid can be added in Firestorm simply by opening preferences and clicking on the OpenSim button, then just add the following mk3.org:5002 and then click Apply.