Activating Mesh Paint Mode
In order to summon and use the Mesh Paint tool, you will need to enter Mesh Paint Mode. Just click the paint brush button on the Modes panel.
This will show the Mesh Paint panel, which remains docked within the Modes panel of the editor frame.
While in this mode, you can still perform most common editor actions like camera movement and selection, but certain features are disabled. Also, you will notice that your perspective viewport will be forced to real-time mode which is a requirement of this system.
To exit Mesh Paint mode, you may simply choose another editor mode within the Modes panel.