The convent of the Benedittini di Tuzzi is on via dei Bevilaqua. It is about 13km from Termini station or about 7km from the Mill Hill House.
Option one: coming from Ciampino Airport.
A taxi from the airport to the centre of town is €30 (price fixed by city council). There is, however, a few shuttle buses that go from Ciampino to Termini Station from where you can get a taxi. You can try Terravision. Their website is www.terravision.eu. For €14 you can get a return bus to Termini and then pick up a taxi. You can pre-book tickets on line or you can get them in the arrivals hall at the airport.
Option two: coming from Fiumincino Airport.
If you arrive in Fiumincino then there is a regular train service to the centre. You need to get the train for Trastevere. The final destination of the train is Orte. Don't get the train for Termini! Everything is well sign posted and everyone speaks English, so ask when you buy your ticket if you need to. The price of the ticket is €5.80
From Trastevere Station take a taxi to via dei Bevilaqua.
The more adventurous can change trains at Trastevere and take the train for Civitaveccia and get off at Roma Aurelia train station. It should cost €1. It is then a 400m walk to the convent.
To get into the centre of town you can get the train to St Peter's which will cost €1 from Roma Aurelia Station. There are also buses from via Aurelia that take you into the centre past the Vatican. Bus tickets cost €1 and are valid for up to 75 minutes.