I'm not familiar with PowerSim, but it is available from High Performance Systems Inc., 45 Lyme Road, Hanover NH 03755. Phone is 603-643-9636, FAX is 603-643-9502. AppleLink address is X0858. They used to have (and hopefully still have) a free demo disk which is fully functional except for saving models. Is a Macintosh (only) product. It is very graphically oriented. The latest version has some "authoring" capabilities as well. It costs about $300, depending where you get it. It is the latest reincarnation of Systems Dynamics modeling software. The original way was Dynamo (on mainframes) which gave birth to MicroDynamo (on both Apple II and IBM computers), and then for the Mac. It is much more graphically oriented, both in its input and output, than its parents.
An "entry level textbook" on the topic is: Roberts, N., Anderson, D., Deal, R., Garet, M., & Shaffer, W. (1983). Computer Simulation: A System Dynamics Modeling Approach. Reading, MA: Addison-Wesley. This book was written for classes using MicroDynamo, however. It may also be out of print. It has examples in ecology, energy, political science, economics, health, sociology, biology, chemistry, and business. It is actually written for a high-school or lower college level class.
One of the manuals that comes with it is called simply Application and gives examples of SD model building in biology, psychology, economics, earth science, and physics. Some of the older manuals from previous versions give examples in other disciplines as well, such as health sciences.
Lastly, look at the more complete version of Mike and Paal's paper on Mike's WWW page for a list of references, including one of the classic books by Forrester on the topic.