1. You ordered something, and you don't know what you're getting for your money? Typically, airbrushes will come with at least a color cup. Others (like Paasche) come in sets with color cups, spray bottles, etc. You should contact Dixie to make sure you get what you need and want.
2. You can use water to thin most acrylics; however, water has a natural surface tension to it (just look at water on a freshly waxed car), and will probaby not go on smoothly. If you want to use water, then add one SMALL DROP of dishwashing liquid to break up the surface tension. You can also use isopropyl alcohol (IPA) that will work better than water. Regular rubbing alcholol works perfectly. Personally, I would use the thinner recommended by the manufacturer for the best results.
3. Run some of the manufacturer's thinner through the brush, and/or some IPA until it comes out clear through the nozzle. I also like to use pipe cleaners saturated with several drops of thinner and run them through the works, and follow with more thinner. It's probably also not a bad idea to take the brush apart at the end of each painting session and clean the needle, nozzle and associated parts. Try to follow or exceed the 'brush manufacturer's recommendations for optimal performance.
Hope this helps you. I have a Paasche but would like to try an Iwata. Let us know how it goes!
Gip Winecoff