Do you have the pro version or standard version of slick? There are some differences regarding macros.
The st command compiles but doesn't load a macro file. Replacing an .ex file in the macros folder will have no effect because loaded macro .ex files are bound into the state file (vslick.sta) in your configuration folder and the .ex file itself is ignored. Slickedit doesn't care what folder a macro source file is located in - all macro source files must have a globally unique name. If you copy briefutl.ex to a new folder and load it using the load module command in the macro menu, it will replace the existing briefutl in the state file. If a hotfix involving briefutl.e comes along, it will replace your version of briefutl.ex. When slickedit starts, if there is no state file, it will probably try to build a new one using the .ex files.
Try recording a macro, then bind a key to it. Recorded macros are stored in vusrmacs.e in your configuration folder and are loaded into your state file. If you have a definit function in your macro file, the definit will be executed when slickedit starts.