I would start again. I would snip off that dying leaf, remove the cutting from the soil, check for roots, check for stem rot, place in a pot with good draining soil, and make sure the pot has holes on the bottom for drainage. If the stem on the bottom is soft and has rot , you will need to cut straight across the bottom until you only white firm flesh and absolutely no trace of brown. Allow the cut end to dry and callous over (about 2 weeks). After the 2 weeks is up, you can place the cutting back into moist soil but don't water again until you get big leaves. Autumn is not the greatest season to start a cutting depending upon where you live. You might want to bring your plant indoors to protect it from the elements while it is rooting or at least to a area which it will be protected from cold temperatures and rain.

There are many ways to start a cutting. Here's an example from a more experience plumeria grower:

http://forums.gardenweb.com/discussions/2116765/best-and-quickest-rooting-method?n=11