Well first off pipettes (paint droppers) are dirt cheat on Amazon. 150 for $8. I recommend you pick some up or some paint drop bottles to store mixed paint in. You should also get a glass container like a shot glass or small jar. It can me used as a mixing jar to mix your paint in.
Well unfortunately you can’t adjust your air pressure with that compressor so you’ll have to play are with the main mixture to get it to flow completely. Since your also using a .3mm needle you will need to thin your paint more so it flows smoothly through the airbrush. Again it will take some experimenting to find the proper ratio.
Now as for a thinner since your using acrylic paint you should be able to us distilled water from your local grocery to thin your paint without any issues. The mix ratio should be 1:1. Meaning for every drop of paint you use the same number of drops of your thinning agent. When you mix it you want the consistency of milk. Since I do not have any experience with Apple Barrel Paints you might have to add a little more or a less thinning agent to your paint to get a milky consistency.
Use the pipette to transfer paint from the bottle into the “mixing jar” the add your thinner. Mix thoroughly. You can buy a badger electric paint mixer. These work great I use mine all the time. Once mixed splash a little pain on the side of your mixing jar. It should look like milk as it runs down the glass. If it does use the pipette to put paint in your airbrush and test how the paint flows through your airbrush.(I have a piece of plastic styrene sheet that I use for testing paint). Your paint should flow smoothly and lay down smoothly onto the your test piece. If the airbrush sputters and has a hard time spraying then you need to thin the paint more. If the paint is runny when you spray the paint you need less thinner. Weather can also affect how your paint sprays. Once you understand the basics of using a airbrush it’s all trial and error to find what works best for you.
I hope this helps.