Your e-shop check box should have a data type of boolean (True/False). Set the default value to false. Lets say that box is called eShop.
Then on each text box, right click each one and under properties click data valadation. Then once that brings up a screen, click Add.
Within that screen your if statement shoudl be: eShop is equal to TRUE(select this don't type) then you can type a message in. that will require that field to be filled in. If you need somethign specific to be in that field or formatted a particular way, create another validation rule.
Repeat for second text field.
http://cid-7a987d5d6da9cbd6.skydrive.live.com/self.aspx/Public/eshop.JPG