There are three different types of WordPress blogs, and only one allows you to upload custom themes. If you’re having issues, hopefully this video will clear things up.
Related videos:
Installing a purchased WordPress theme: https://www.youtube.com/watch?v=nJ0uTu4Vxkk
6-Minute Website with Bluehost and WordPress: https://www.youtube.com/watch?v=41ElWtTpOeo
Blog post mentioned in video: http://novicenolonger.com/how-to-install-a-new-wordpress-theme/
