Once you have Wordpress installed in your blog, follow this step:

• download Wordpress theme, and extract the zip file
• open your FTP client. For example, if you are using Firefox, FireFTP (firefox add on) is highly recommended.
• connect to your user account, and open /yourdomainname.com/wp-content/themes/. Drag the theme folder here.
• Now go to your dashboard, and choose Presentation >> Themes. Activate your theme.
• Done!