I need to buy Maas IFF code from the Junkyard. What I have to do/steal to buy from them as they ask me to get more stuffs/cargo.

Ok Im not quite sure exactly WHAT you have to bring them, but Ive found that any piece of cargo works, ive done it with everything from brandy to mood enhancers. All you have to do is go to the "hangar" section of your menu at Lucrecia's Base and click on the "add cargo" button, launch w/ your cargo, go to the Junkyard, and when they tell you drop it ("U" key) and clay should come on w/ info about the Maas IFF. If u need any more help w/ that mission email me and ill give u the info u need.

I tried yeast or whatever its called and they did actually say that wasn't good enough.

Further to this, what on earth am I meant to do when I get to the garbage area and it says there is a useful pod and the drone. I don't get it.

You hame to rem-link to the drone. Then use the drone to dock with the interesting cargo pod. take this to the ship pod and switch it with the one you have. Then return back to your ship and release.
Your free to return home with your stolen cargo.