Eh? If the theme had *BaseModuleHook !vTrayHook it needs to be replaced with *BaseModuleHook !xTrayLSBoxHook. Hooking generally requires 3 things. The module's line command (in this case xTrayLoadInBox), the hook/bang command (in this case *BaseModuleHook !xTrayLSBoxHook) and that the (x)label module be loaded AFTER the module being hooked (in this case, xtray needs to be loaded before xlabel). I'm thinking that last thing could be your problem, if you just appended xtray-ver# on to the end of the module list. It has to come before xlabel. As for where to look... open all the files in a good, tabbed text editor, and just do a search for the right word.

Oh, and keep in mind, once you get the module hooked, that all of its movement/location settings become relative to the inside of the label itself.