why do you want to hook it up to the serial adapter? does the power pin on your etrex cable connect to that? i don't think it's a good ideal to connect the 5V USB power line to that. it's safer to separate it. hack your etrex cable instead to isolate the pin 8 from the serial adapter and then hook the power & diodes to the etrex side of the cut.
i built my own cable using an etrex connector from pfranc.com and then hooked it to a 5V ipaq cigarette lighter charger via about 4 series diodes (sorry i don't remember the details because it was 3yrs ago, but it's always a good idea to measure the voltage under load and add diodes until you get as close to 3V as you can).
it appears from the link you sent that the guy hooked up the zener diode in series. that's not the right way to do it. zeners are supposed to be hooked up as a shunt. here is a link to the proper circuit...page down to "Voltage Shunt Regulator."
http://www.itee.uq.edu.au/~engg1030/...age/lect10.pdf
personally i like the series diode idea better because it doesn't waste power thru the shunt and load down the USB output needlessly (less chance of frying yr laptop). the only problem w/ the series diode circuit is the voltage fluctuates a bit w/ different loads so it's impt to put enough diodes to get the loaded voltage down to 3V. if you don't have a voltmeter start w/ 6 diodes if the etrex is flaky remove them one by one until the etrex runs stably.