Go to the 'Filtering > Domains' tab and add the following domains to the whitelist, set to 'bypass filtering and authentication'


in WebTitan version 4.n
.apple.com
.itunes.com
.mzstatic.com


in WebTitan 5.n

apple.com
itunes.com
mzstatic.com

==> make sure Include Subdomains" is check


Then go to the 'Filtering > URL Keywords' tab and add to the whitelist

ax.init.itunes.apple.com

set this to 'bypass for domain only' and 'bypass from filtering and authentication'