Your terminology is confusing. Where is the subdomain? Is "zen_cart" just a
subdirectory under the domain, or is there a real subdomain or add-on domain involved?
Assuming you have only
www.skybyes.com, are you saying that you want URLs without any path or file to go to
zen_cart/, and everything else be left alone? That should be easy enough. I think this should work:
RewriteEngine On
RewriteRule ^$ /zen_cart/
I don't think you want a 301 (permanent) redirection, as Google et al. will stop listing
www.skybyes.com and list only
www.skybyes.com/zen_cart/. If you do want that, add
[R=301] at the end of the RewriteRule.
By the way, you may want to consider changing 'zen_cart' to 'store' or 'shop' before you go live -- it looks more professional that way.