Firstly if you are upgrading from a pre version 2.1.3 template you must follow the upgrade instructions so you have copied the "ContentSearch" document into your blog database.

Then follow these steps:
1.        Add the <$DXSearchForm$> tag to your site template where you wish the search box to appear.
2.        Goto templates in your blog client - select search templates and create a new template giving it a name.
3.        Copy the html from your site template into your search template to give you a starting point - replacing the <$DXContent$> tag with <$DXSearchResults$>.
4.        Goto your configuration document - templates tab - select your search template in the search template drop down - and save.
5.        For performance make your blog database full text indexed.

For additional settings there is also a search tab in the configuration document - further documentation on this soon.