Your snippet will be populated based on the information Google feels is the most relevant to the query. Meta descriptions are one of the elements Google can use for this purpose, but if it feels your on-page content (for example) is more relevant to the user's search, this will show instead.
The Open Directory Project is another way your snippet can be created - Google uses DMoz and other listings to create your snippet, although you can opt out of this by using the noodp instruction in your robots meta tag.
If you want your carefully-crafted description to show in search, make sure it is directly keyword-relevant to the query you want it to show for. If you're selling red shoes, make sure 'red shoes' is included in the meta description, preferably as an exact match phrase. This won't help you rank higher, but it will help your MD show as opposed to your page content or any other information.
