The conference on commutative rings, integer-valued polynomials and polynomial functions is located at the department of mathematics, section A of TU Graz (Technische Universität Graz) , Steyrergasse 30.
How to get to Graz from Vienna airport (Wien Schwechat - VIE )Take streetcar 6 in the direction to "St. Peter" and get off at "Neue Technik", at the corner of Steyrergasse and Münzgrabenstraße. The conference location, TU Graz math department, is then about 150m to your left, at Steyrergasse 30, and Hotel Johannes about 50m straight ahead, at Münzgrabenstrasse 48. (For hotel Mercure Mess, continue on streetcar 6 for two more stops and get off at "St. Peter Friedhof".) To get to hotel Mariahilf from the train station, you also take streetcar 6 in the direction to "St. Peter", but get off earlier, at "Südtiroler Platz", the hotel is at Mariahilferstraße 9, about 100m down Mariahilferstraße on your left.
You can buy a single ride streetcar ticket from a machine on the streetcar. Also, you can buy 10-ride tickets for the Graz public transportation network at the ticket-vending machines in the train station and at tabacconist's. One ticket is valid for any trip of at most 1 hour inside the zone of Graz (which includes the airport) with any combination of streetcars and busses.
Here is a public transportation map of Graz. The website of Styrian public transportation can be a bit confusing.