You'll want to draw your walls--internal or external--as lines. There's no explicit wall thickness modeled in Sketchup for translation to Openstudio, like shown in the following video. The walls' thicknesses are determined by the construction sets chosen, which can be based upon ones provided in templates/libraries per climate zone and standard. These libraries contain construction assemblies for Exterior/Interior Surface (Walls/Floors/Roofs/Ceilings) Constructions, Ground Contract Surface Constructions, structured as shown in the Constructions tab. Alternatively, you can build your own constructions/construction sets and apply them to your building. If you need to account for wall thicknesses at a window like you would in daylighting, this is done via the OS:WindowFrameAndDivider as noted in the 1.7 release notes.