From Memmingen Airport to Munich City Centre

2 hours, maybe 25 euros; this ticket is not fully sold online, so buy in the station. How to Look Up Train Schedules and Routes Online gives you the DB train schedule link and tips for using it.

would anyone know if website is from memmingen airport to munich city centre?

The Memmingen airport is not actually in Memmingen. It is in the small town of Memmingenberg, near Memmingen. The are no trains to Memmingenberg, only buses.

Although there are trains from Memmingen to Munich, you must first take a bus (3€) to Memmingen and change there to the train. The trains from Memmingen to Munich are either regional (1½ hr), for which you can not get reservations, or express trains (EC/IC, 1¼ hr), for which reservations are available. You can buy a full fare, single trip ticket for the regional train for 23,20€, or buy a Regio-Ticket Allgäu( pass) for 20€. The full fare single trip ticket using an express train connection is 26€; a non-refundable, train specific ticket for the trip using an express train costs 19€ if purchased far enough in advance.

There are also (per your website) buses directly from Memmingen to Munich ZOB for 8€. (Munich ZOB is at Hackerbrücke, from which it is a short, 1,40€, S-Bahn ride to the Hbf). If you take the bus from the airport to Munich, you should also purchase an MVV Single Tageskarte for the Innenraum (inner zone of Munich) for 6,20€ to get around in Munich (including the ZOB to the Hbf) after you get there.

Note: the Regio-Ticket Allgäu is an "all day" pass valid in 2nd class for all regional trains (RB/RE/IRE/ALX etc) in an area roughly between Ulm, Munich, Lindau, and Füssen. It doesn't appear to include S-Bahn, so you could not use it for travel inside Munich. For 3€ more, the Bayern-Ticket includes regional trains in all of Bavaria, plus all conveyances (S-/U-Bahn, trams, and buses) inside the MVV (Munich metro district).