If you want to purchase the voucher on line prior to your trip, go to the TIM website. This will land on a page in English and you can read the explanations but the "BUY" button does not work. Switch to the Italian version and it does. (I just bought mine.) If you have a problem with the language, cut and paste it into Google Translate.
Or just use the Google Chrome browser which will translate the Italian automatically.
30 Euros for a month of data is a decent deal (especially if you need a lot of data - 15GB is way more than I would use on any trip to Europe), but if you go to Europe once a year or so you can just get a SIM that you can keep alive and re-use. I used my Dutch Vodafone SIM again in Portugal in May - cost me 10 euros for 2GB of data (which would last me about three weeks). It can be kept alive by adding credit or sending a text about once every six months. The TIM SIM makes more sense if you need to make locals calls in Italy and want an Italian phone number, but I really haven't needed that.
The FanDom blog has an interesting blurb on this SIM:
http://prepaid-data-sim-card.wikia.com/wiki/Italy
For tourists TIM has released a SIM card called Tim Tourist for 20 € (+ 10 € for the SIM, including credit of 1 €), 200 minutes for call and 15 GB of data in Italy and up to 6 GB in the EU for a month.
Calls are domestic and international to 65 countries (for list scroll to Paesi Abilitati), data is on 3G/4G and will be throttled to 32 Kbps after exceeding quota. No upsize or extensions possible.
Some shops may not like to sell you this offer, as they don't get any benefits from it. Be firm and polite. You may show them a print-out of the offer or let them call Head Office. Numerous users reported problems in getting this official offer by going to more than one TIM store, but it's possible.