Tumblr offers dozens of free themes for your blog; themes are user-made, and don't always include the same attributes. As such, some themes don't include the description block. If you've filled in the description section on the customization page and there aren't any HTML errors (such as open tags) that would cause the text not to appear on your page, then you can either change your theme or edit your current one.

Choosing a Tumblr Theme

Many free Tumblr themes do include the description section, either in the header of the theme or on the sidebar. Avoid single-column themes, as they're more likely to lack the description. The best bet is to look at two-column themes, which usually include a traditional sidebar.

Modifying Your Theme

You can modify your theme to include the description section. On the customization page, click "Edit HTML" by the theme thumbnail. Find where in the theme you want to add the description and add the {Description} attribute -- brackets and all. You can enclose this in paragraph tags or div tags and theme it however you like.