From Cologne Mainstation take the Bus #133 in direction Frankenstraße to Heumarkt. Change there in Bus # 106 in direction Marienburg / Südpark to station Schokolademusem (is next stop).

You will reach the Schokoloadenmuseum in 5 minutes from Cathedral, but only if you are a really good runner , so expect a walk of 20 - 25 minutes along the banks of river Rhine.

You might be interested in the link to the public network: www.vrsinfo.de and in an info about a pdf-file about the Cologne Bus Network.