The critical aspects of roughing are as follows.
Getting to depth There are two aspects to getting the keyseat cutter to depth. The tool must first move down in Z, and then it must plunge into the metal. For the down move, the keyseat cutter simply plunges down. You must have air underneath the cutter for this move. You may set the predrill point to determine where the keyseat cutter is lowered in Z. Then the tool plunges into the material horizontally. There are no options to control the plunge into the material.
Vertical step No vertical steps are taken. You get a tool selection error if the tool width is not exactly as wide as the groove.
Horizontal stepover Rough pass stepover can override the horizontal stepover into the groove. Otherwise FeatureCAM automatically takes as many steps as it needs to cut to the full depth of the groove. 33% of the tool diameter is the default.
Finish allowance controls how much material to leave for the finishing pass. By default this is 0.02.
See also How is an ID or OD groove machined? and FeatureMILL 2D Milling algorithms.