This company makes trail maps that cover the whole country.
http://www.singletrailmap.ch/singletrailmap/index.php

Mountain Biking in Wengen and Muerren

A print map is available at the Tourist Information Center, but the wengen-muerren.ch site has a nice set of jpg format maps:

• Interlaken-Lauterbrunnen and Interlaken-Grindelwald
• Loop trail around Mürrenterrasse
• Loop trail around Männlichen
• Männlichen
• Kleine Scheidegg
• Bussalp/Waldspitz
• First/Grosse Scheidegg
• Rund ums Faulhorn

Mountain bike rentals are available at:

Central Sport, Wengen +41 33 855 23 23

add shops from here
http://www.streetstepper.com/dealer-overview/ch
River Bike 8260 Stein am Rhein Switzerland
Wenger 2-Rad Shop 4052 Basel Switzerland
Haltiner Titus GmbH Velos-Motos 9462 Montlingen Switzerland
feel your body gmbh 8610 Uster Switzerland
Beat Metz Zweirad 7270 Davos Platz Switzerland
Strasser Velo-Motos AG 8524 Uesslingen Switzerland
zweirad shop harry hintermann 8608 Bubikon Switzerland
Roleto BiKes 8834 Schindellegi Switzerland
bikeimpuls GmbH 2552 Orpund Switzerland
Allroundservice Urs Tinner 9469 Haag Switzerland
Walter Meier Velos und Motos AG 9400 Rorschach Switzerland
Max Bersinger AG 9015 St. Gallen Switzerland
2-Rad-Fachhandel Brauchli & Co. 7302 Landquart Switzerland