Find green eco fuel is the first mission in The Lost Frontier. After crash landing on Brink Island, Keira tells Jak and Daxter to go scout the area for more green eco fuel to repair the Hellcat, though not before handing him the Gunstaff.
The primary purpose of this mission is to get reacquainted with Jak's moveset, identical to that of previous games. There are a few differences, however, including a loss of rolling and roll-jumping, a reduction in the fluidity of Jak's movement, and an extended range of Jak's punch.
Proceeding with the mission, Daxter will guide Jak across a few jumping segments and through a fairly linear area before approaching a band of killer armadillos (specifically of the Brink variation). Ahead of these stands a large stone pillar, which you will be prompted to knock over (using successive and presses) to create a bridge.
Armadillos will continue to appear throughout the rest of the mission, which will remain fairly linear. The armadillos will often burrow underground, travel, then reemerge before rolling up into a ball and launching themselves toward you at a fairly high speed, which is their main attack (aside from simply biting you at close range). Their roll attack is rather easy to avoid so long as you see it coming, though a lack of camera control can sometimes reduce your visibility.
After finding the eco crystal, return the same way you came to bring it to Keira, defeating more armadillos as you proceed.