Start by researching how to build Palm OS applications in general. Such applications (provided you restrict yourself to the 4.1 APIs (easy to do)) will run on the Alphasmart, just in a square area.
Eventually you will find yourself looking at the Access site. Access now call Palm OS Garnet OS for they no longer have the rights to the Palm trademark. You can use the tools and SDKs found there, just pay attention to the documented supported OS, ignoring those in 5.X.
Apparently the AlphaSmart SDKs have disappeared for download. They actually where not much, mostly just documentation on how-to. There is some discussion of some of this in this thread.
Linear Mode
