For offset curves from edges see Reference Geometry (bottom of page)
Selection
You may have to experiment to select the appropriate quilt for your offset feature, as original surfaces are processed though merging and trimming etc. they take on new identities.
To select a single patch you may have to select the quilt then pick again to work down through the geometry to the required element.
Also RMB > Pick from list
Offset
Set your offset parameter and preview the geometry to force processing and consider any special handling needed for problematic quilts.
Options tab > create side surfaces to create a set of surfaces for a volume – the three quilts (original, side surfs, new offset surf) will need merging before they can be solidified.
Special Handling
As with shells and thickens there will inevitably be surfaces for which the system cannot create the offset geometry – small surfaces, surfaces which are consumed due to the offset dimension.
Always 
preview the feature to force the system to attempt a resolution, if there are issues you will generally be offered options for handling the problem surfaces. Alternatively you can manually exclude surfaces you think will be an issue.
CADCAM Resources 