Just got it to work! To enable the Mesh Enabler plugin in Inventor you need to press the +Add Ins button on the toolbar and then set the plugin to load automatically.
After that bring in an STL file, use the Fit Mesh command to generate surfaces for the holes. Then right click on the mesh in feature tree and select ‘Convert to Base Feature’. Then export as STEP and bring into Solidworks. Now the STL files can be assembled using the surfaces for hole mates for hardware! 🙂