Here is a link to a site showing various wiring diagrams as to how a three way can be wired in different situations. Unless you"ve got a neutral at the switch your trying to tap into there"s no way to "legally" wire in a receptical like your wanting to do.
In any case you can look a the diagrams and see that the black wire is a power wire and the red wire is also a power wire called the traveler. Because both carry power and neither is a neutral you can"t make an outlet work off of those two wires alone. Fortunately the twowires carry the same phase so you don"t wind up with 240 Volts across them when you hooked them to opposite sides of an outlet. Too since the voltage is basically dead headed when the two are connected through a load they shouldn"t blow up anything, otherwise what you plugged into the outlet to test it would have been ruined.
The best thing to do in a case like this, especially when your not sure what"s going on, is to get a qualified electrician to do it for you. Until then places like Home Depot, Lowes, etc still carry "inline outlets" that are made where you can screw it into the light socket and have an outlet or two and still screw the bulb in also.