Hopefully you unzipped the template before you FTP'd anything....
If you had, you would have noticed a file in the folder with a "
.txt" extension .... this file contains the full installation instructions, which you are to follow precisely.
And, just a little "heads-up" on the process of 'skinning' a cart, a forum, or anything else: you do not
move files and folders via FTP, but
COPY them to the proper destination. This is your only hope of keeping a non-corrupted file in your possession, should you need to re-load your site for any reason.
If you are having particular difficulties skinning your cart,
use the link in my signature below to find the Official osCommerce Support (forums, etc.) that will have the pefect solution waiting for you if you Search the forums either by reading or using the Forum Search tool.
You might also wish to see what degree of installation support is available from the provider you purchased your template from. (They all offer installation help)
Professional Template Installation & Customization