To build your own LED tester, all that's necessary is to use a current limiting resistor so the LED does not burn out. A simple explanation is at
http://www.instructables.com/id/Choosing-The-Resistor-To-Use-With-LEDs/ and all parts should not cost more than $2. All you are doing is counting the number of pulses to get the error code, so this should be decently simple if you are familiar with electrical work. If not, find a friend who is able to give you a bit of assistance in the electrical/electronic area, you really don't want to damage the expensive engine control unit...
Also search "ABS" in this forum, and Google "LED" for more info on building your own cheap LED flash/test circuit... Hope this helps you a bit...
Edit: As the instructions at
http://bmwk10075abs1fix.web.fc2.com/ state, ensure you use a high impedance (resistance value) for your LED tester so you don't overload the ECU, or you can use an analog or fast acting digital multimeter to get the same results (number of counts). Sorry if this sounds a bit obscure, have worked around electronics for many moons, and unfortunately have "let the magic smoke out" a few too many times...
