Denmark is a small country, so as soon as you reach the country, you are as good as there! The three appropriate airports are:
From Kastrup you can either go by domestic flight to Tirstrup (45 min) or take the direct train to Aarhus City (3 hours). From Tirstrup busses are coordinated with the flight schedule and it is a 45 min busride directly from the airport to either Aarhus City or Aarhus University (same bus, different stops). From Billund it is a 90 min busride to Aarhus City.
Note that taxis are expensive in Denmark! If you take a taxi to Aarhus from Billund Airport, it will cost you 1700 DKK (~300 USD), and from Aarhus Airport Tirstrup it is 500 DKK (~90 USD).
Trains arrive in Aarhus City from Copenhagen and Hamburg. At some departures you will have to change trains in Fredericia or a station close to Fredericia.
Motorway E45 that runs through Europe (under different names) passes through Aarhus. The road crosses motorway E20 in Kolding, make sure to go north from there. When you reach Aarhus, keep right and follow route 501 (towards 'O1'). Turn left at 'Skanderborgvej', and left again at 'Søndre Ringgade' (O1). 'Søndre Ringgade' turns into 'Vestre Ringgade' and then 'Nordre Ringgade' which is the north perimeter of the university park. At the cross of 'Nordre Ringgade/Vestre Ringgade' and 'Langelandsgade' you will find a huge building (relative to the other buildings in that area) which is 'Statsbiblioteket'. The venue (Department of Chemistry - "Institut for Kemi") is located on 'Langelandsgade 140, 8000 Aarhus C, which is a two or three minute walk from 'Statsbiblioteket'.
It is expensive to rent a car in Denmark, see e.g. rentalcars.com for prizes.
Say, you have reached the railway station either by flight bus, train or feet. If you continue on your feet, the conference venue is 30 min away.
By taxi, it will take you 5 minutes and cost about 120 DKK (20 USD).
If you choose to go by the yellow city busses then these two numbers will take you closest to the venue:
They both leave from 'Park Alle' right next to the railway station (which is at 'Banegårdspladsen 1'), but it can be difficult to find out in which direction the bus is headed. A bus ride (2 hours) costs 20 DKR, but in almost all kiosks and supermarkets you can buy 10-ticket coupons (klippekort) for 140 DKR.
Read more about the busses here.
You can also use the city bikes if you can get hold of a 20-crown coin. Read more here.
Scandic City, Scandic Plaza, Hotel Ritz and Cabinn are all located in the city center, in walking distance from the railway station. The yellow city buses no. 2A and 16 (see links above) will take you from the hotels to the university. The bus stops located closest to the hotels are marked in the linked files.
Danhostel Aarhus (Vandrerhjem) is located just outside the city, in the woods of Risskov. Buses towards the university departs from Marienlund. Bus 5A goes straight from Marienlund to the university (and back).