Going by this [Official Google Blog] meta description tags are helpful to search engines.
[Update - Sandvox 1.6.x will do this from the Site Outline, Meta Description box]
Prior to 1.6.x this could be added to a Sandvox Pro created site using code injection.
The Google Blog linked to above recommends the description to be different on each page, therefore use page code injection:
Site > Page Code Injection... > Head area : Top box
Screen capture: -->Meta Description Placement
