The 50 credits you've acquired were from your promotion from Hack to Amateur. If I recall correctly, you'll earn 100 for turning from Amateur to Pro, and 200 for turning from Pro to Master.
There are many other ways to earn credits. The first of which is to Buy Credits, all you have to do is link a credit card or paypal account and you can deposit funds.
The second of which is to Complete Offers. WGT is linked to many sites where you can sign up [sometimes for a charge] and get credits here on WGT by signing up there. Some of these sites include Netflix, CreditReport.com, GoDaddy.com, and many others. You can earn up to [I believe] 5,550 with the completion of one offer.
Many weekly tournaments also offer credits as a prize. Check out the Tournaments Page and look to see what the prizes and payout rules are. Some tournaments offer a prize for the top finishers, and some are randomly drawn [Closest to the Hole]! So even if you're not confident in your ability, you can still play and have a chance.
The final way is by participating in Ready-Gos. These single play stroke play tournaments cost 100 credits to enter and pay off at 5,500 credits for first place! Pretty good if you ask me. :)
Good luck and happy hitting!