You will need to clear the server cache by touching the Web.config or recycle the app pool, then clear the browser cache.

The stylesheet will not be attached, it will be combined into one request with all the other css files listed in style.config.

While you are designing its best to disable the caching by set these to false:

<add key="CacheCssOnServer" value="true" />
<add key="CacheCssInBrowser" value="true" />

but afterwards best to re-enable caching.

Hope it helps,

Joe 

You can set this to false:
<add key="MinifyCSS" value="true" /> 

Then view the source of the page, get the url for the ..csshandler.ashx?... and paste it into the browser and determine if it is in fact including your css.

Make sure you are doing all this in the skin folder actually being used /Data/Sites/[SiteID]/skins/[skinname]

Hope it helps,

Joe