MacOS 7.1 or newer is required. Both PowerPC and 68k-based Macs are supported. A fast machine (at least 40 MHz) with lots of RAM (probably at least 24 Meg) is recommended. Disk space needed for Grail itself is negligeable; you will need several Megabytes for Python and Tcl/Tk each.
You must have properly configured networking. If you've never run another Internet application on your machine, this likely isn't the case. If you're using AOL or a similar "closed system" application, this may or may not be the case. If you can use a "telnet" application, you have the right kind of networking.
Download and install the latest installers for your platform for Python 1.5 and for Tcl/Tk 8.0. Newer releases are usually fine, but stay away from alpha or beta releases. Tcl/Tk uses "patch" releases with numbers like "8.0p2"; use the highest patch release you can find. Download locations:
To download Grail, you need to agree to the license: