The ReactionDiffusion component combines a MeshedDrawing with a ReactionScheme and computes the evolution of the concentrations of the species present in the scheme.
You can construct semi-permeable boundaries, apply injections, and specify distinct initial conditions in separate parts of hte grid. Most if this is done on the meshed drawing iteself. The computation is done by the ReactonDiffusionCalc object, which produces a variety of results. The clearest to start with is probably the "play-through" of the concentration as a function of time: click the "calcres" button.
Setting up initial conditions is split between the _EDIT(meshedDrawings,meshed drawing) and the _EDIT(reactionSchemes,reaction scheme). In either case, select "new" from the top left menu to create and display a new object.
In the latter you must build up the sets of initial conditions you want. In the meshed drawing, conditions are set by using an "Init" region marker, with its target point anywhere in the region to which the conditions should be applied. Which set of conditions to use is specified by the "initCondSet" variable in the "Init" marker. this is the index of the set to be used in the list contained in the reaction scheme.