Hello,
if you are using an out of the box SharePoint 2010 template, then default.master is not the correct master page to edit, anyway.
You should also avoid changing the original master pages. Instead, create a new master page and set your site to use your custom master page.
What kind of site are you working with?
Method with CEWP: Put the CSS into a text file. Upload that text file to a document library in the site. Now create a Content Editor Web Part on a page and link to that text file. If this does not work then double check that the CSS is correct and that the
file is loaded.
Master Page method: In SharePoint click the ribbon icon to create a new master page. Then open the v4.master and copy all the code. Paste it into the new master page, replacing any existing code. Now load the CSS in the page header. Save the new custom master
page and set it as the default master page.