Why can't i transfer a theme that i downloaded from the pc to my Nokia N73 mobile?

[Lilo]

I connect the phone using PC Suite and when i transfer the themes on to the phone it says "Certificate error". how can i get the themes on the phone? what applications do i need to download to get it to work? also its a Vodafone phone, and I'm under a south african network..

please help..
When i connect the phone to the pc as "mass storage" then the themes do go on, and its in my file manger, but it doesn't appy, when i click on open it says certificate error again

 

[Vinit]

you can get themes from zedge.com........
nyways u hav to install the themes to ur cell only then can u apply the...nd if u getting expiry certificate error u can go to ur calender change d date few months back and thn install it.....i hope this help u.......