Q: This guide doesn't work in the new PC itunes!

Which part isn't working ? If you have the current version of iTunes (iTunes 12) on your PC then the steps for logging into your account should work.

Or, and if you have an older version of iTunes, you can log into your account via the Store > View Account menu option :

On your account's screen there should be a Purchase History section with a 'See All' link to its right

That sounds like you are going to the Purchased section in the store, not viewing your account. What exactly are you clicking on ?

To view your account's purchase history either click on your name towards the top right of iTunes and then click Account Info on the popup (nor Purchased) :

Or use the Store > View Account drop-down menu option - they both take you to the same place.

If you are now on the purchases list screen (i.e. you've clicked on the 'see all' link) then your purchases should be listed in chronological order, you will need to page through them to find the purchase(s) that you're interested in (there isn't a search function or a way to narrow it down by app) - click the arrow to the left of a purchase line to view the purchase details : item(s) on the order (if I do multiple purchases on the same day then mine tend to be grouped into a single entry on the order list screen), order number, invoice number, price of each item on the order.