Reaktor/Tutorials/Reaktor as effects processor/Signal routing: mono to stereo - stereo to mono
Signal Routing: Mono to stereo/stereo to mono edit
Any audio output can be directly cabled to multiple inputs in any R structure, so it's not necessary to use a mixer or other device to split a mono audio signal to send it to both inputs of a stereo processor, or to send stereo or mono signals to multiple inputs. But when you want to combine stereo or multiple outs into a mono input, or multiple signals into a single stereo input pair, you need a mixer or a signal adder.
Mixers are available as instruments, complete with snapshots, found in various config-durations the Instrument library, and in souped-up versions in the User Library. You need a mixer only if you want level and other controls along with the mixing function. Remember that you can delete or disconnect any unwanted panel controls in the structure; disable, hide, or show different sets on the A and B views using the Properties window; or simply collapse the panel when you don't need to see or set the controls.
Adders (which are simply mixers with no controls and all channels set to 0dB when connected to audio signals) are modules; choose “Add” from the Math collection in the Module library. Unlike Mixer instruments, which can be inserted into ensemble structures, “Add” modules can only be inserted inside instrument and macro structures. Once inserted, you define the number of inputs your adder module has by opening the Properties window (F4-Mac/PC), clicking on the adder's label to focus the Properties window on it, selecting the Function page (click on the “gears” icon), then scrolling to or typing in the number you want in the “Min Num Port Groups” field.