There are a number of designs that you can use to build a JP1 Cable. The easiest and most commonly used design is called the Simple Interface design. The instructions for how to build one is in the ‘0. Simple Interface’ folder in the Files section of the JP1 Users Group. Other, more complex designs are included in the ‘Interface Designs’ folder.
You should get the instructions for building the interfaces from the JP1 Users Group, build the interface, and attempt a download. The instruction manual also includes a number of specific troubleshooting steps to take if you having problems.
Reference Links
http://groups.yahoo.com/group/jp1/files/0. Simple Interface - Link to the directory containing the most recent set of instructions for building a JP1 Cable using the Simple Interface design.
http://groups.yahoo.com/group/jp1/files/Interface Designs - Link to the directory containing the most recent set of instructions for building a JP1 Cable using alternative designs.