Just go to the iTunes store (you'll be familiar with this if you're an iPhone user), download and start using it. It's miles easier if you already have an online account or existing loyalty card; although you can (and I did) register straight from the phone app, if you have any difficulties then the website rather than the phone app is your best place for help.
So why bother? Two words: free delivery. Yes, you can find just about everything on the app on Amazon too - I was checking for the new Armani Code Summer edition eau de toilette for men, which I'm eager to try, and it was the same price but minus a fiver delivery makes a difference.
If I had two wish list items for the app then the first would be that it could stand by itself a little more rather than be quite so tied into the website. For a first "draft" of the app this is pretty good, though. The second is that there could be a less obviously feminine interface available as an option. I had to check with the public relations person (who was highly responsive, 10/10 for her efforts) whether there was actually anything in this for the LifeOver35 audience.
Arguably there needs to be a whole new vocabulary here. Not only is there stuff for blokes behind this feminine facade but it's all classified, in the shops as well as online, as "beauty products". Very few blokes are going to wander into those areas for items for themselves.
And on the strength of this, once we've got past that there's money to be saved.