You will need to setup Citrix Secure Gateway to allow secure access to you site from outside your network.
http://www.dabcc.com/article.aspx?id=12844 is an article I wrote on setting up and testing external access using CSG.
Once you have tested that, create a XenApp services site (PNAGent site) and it will pick up the CSG/SSL settings. THen install the Citrix Receiver for iPad and configure the receiver to acess the PNAgent site. I have articles showing how to do all this that will be coming out July 6th and 7th.