The "Welcome Mat" is available in the "Gadgets" tab of the Palette (in the "Other Gadgets" category). You tap it and pull the trigger on the Maker Pen to place it somewhere in a Custom Room. Visitors to this custom room will then spawn above the welcome mat. You can edit a welcome mat with the "Configure" mode of the Maker Pen and specify a room name for it; in this case, the welcome mat only works when visitors are coming from that specific room.