I don't have a car so I use the LIRR at least a couple of times/week. By taxi, the Greenvale train station is about 5 minutes from campus and the westbury station is about 20 minutes from campus. The Roslyn Taxi company charges $21 for NYCOM to Westbury train station and I think $11 for Greenvale. Roslyn Taxi's phone number is (516) 621-1111. Try to call 1 hour in advance to make a reservation. I use the Roslyn Taxi service for either station. When catching your train back add 20 minutes to the driving time since sometimes the taxis are 5-10 minutes late and traffic in LI sux.
There is a taxi service in Westbury that I used a few times but the dispatcher is rude and they charge about the same I think.
My suggestion is the Greenvale station. Check the LIRR schedule since the trains run about every 30-1hour sometimes with a 2 hour break in the schedule.
LIRR fare from NYC to Greenvale is about $6.50 each way if you purchase the ticket before boarding.
If you take the LIRR from NYC you will have to transfer in Jamaica queens going to either station.
http://lirr42.mta.info/index.asp
Last year we were not really able to predict the exact time/order of interviews so maybe you need to bring that up when you check-in at the admissions office. Last year I ran into a nice group at my interview and since there was no rush to get home, we waited for each other to finish interviews and shared a cab to take LIRR.