Cologne is the fourth largest city in Germany. It is located on the Rhine river right in the middle of Europe. For general information on visiting Cologne, have a look at KölnTourismus or at koeln.de. You might also want to check out the Cologne wikipedia page or the wikitravel page.
Cologne has its own international airport, Cologne Bonn Airport (CGN) and can be reached in about 40 minutes by train from Düsseldorf International Airport (DUS). However, most transcontinental travelers might find it more convenient to fly to Frankfurt Airport (FRA), Germany's largest international airport. There are fast ICE trains from the airport to Cologne central station that take about 50 minutes. Some of these trains can be booked as Lufthansa flights together with your plane tickets (the airport IATA code for Cologne central station is QKL). Several airlines also offer flight and rail, i.e. combined tickets via other German airports.
The Cologne central train station (Köln Hauptbahnhof) is one of the biggest German train hubs with good connections to France, Belgium, the Netherlands and the UK. It is located right in the city center. For connections and time tables check out bahn.de (also for international trains).
For those going to PKC 2012 in Darmstadt after Pairing 2012: Darmstadt can be easily reached by train via Frankfurt main station or via Frankfurt airport, see the PKC 2012 travel info page for further details.
The Marriott Hotel, Cologne is located right in the city center and can be reached easily. It is just a short walk from Cologne central train station (see map). Simply exit the train station on the side opposite the Cologne Cathedral (Kölner Dom) and walk 5 minutes along Johannisstrasse.
For information on visa regulations for visiting Germany, please consult the visa regulations page of the German Federal Foreign Office.