Unlocks at level 40 as the description says and will come with natural progression the more you play. If you want to speed things up a bit, I suggest playing as a support class e.g. engineer/support as this game tends to reward players for working as a team as opposed to going for kills.
You can rack up a lot of points by just hanging back and creating nodes etc then dismantling them and rebuilding. Just try not to annoy your team in the process.
Defence games are great for gaining XP additionally play as support and just help reinforce the current strongpoint by building nodes and obstacles.