To fix your cghain not going into the bottom (largest) cog at the back, unscrew the little knurled screw when the cable goes into the derailler at the back 1/4 or 1/2 turn. This will compensate for cable 'stretch' (settling in to the cable stops). This is normal in the first couple of weeks use. When you are riding in the middle chain ring look down to see if the chain is rubbing against the inside or outside of the cage. If it rubs on the inside, tighten the gearshift cable a bit. If it rubs on the outside, loosen the gearshift cable a bit.