We do not upload themes here at wordpress.com. The uploadable themes you are looking at are for wordpress.org and they run a different software. There is a pink sticky post at the head of the forum “please read me first before posting” that explains the differences between the two http://wordpress.com/forums/topic.php?id=3700&replies=1
To find the choices for themes you can use here at wordpress.com go here -> Dashboard -> Presentation -> Themes (click on the theme you choose and it will appear on your blog).
Also as you are new to wordpress.com this will answer any questions you may on editing themes at wordpress.com and on upgrading and undertaking css customization of a theme.