Hi Flyinghigh65,
I use humbrols all the time. If you are going to put any of their paints/matt,satin,gloss cotes through your airbrush you definitey need to thin them down.
You can use any enamel thinner to to the job, I use artists pure turpintine. If you decide to use thinners, remember that the fumes smell bad and can be harmful!
Please make sure you use in a well ventilated area!
Hope this helps,
Darren