Addon Domain, WordPress Not Reconizing New Themes

Ijust added a addon domain. I have installed WordPress. It seems to be working fine. I can write posts, I can receive comments. However when I upload new themes WordPress doesn’t reconize the files? Can anyone help me or point me in the direction to be helped. ON my main domain I have a fully functional Wordpres Blog which I have customized, so I don’t feel like I am complete idiot here.