Shimano derailleur gears

We have two Dahon Boardwalk folding bicycles fitted with six Shimano gears changed by a MRX twist grip numbered 1 to 6.
Unfortunately on both bikes the number on the twist grip does not correspond with the right gear by one gear. That is if the twist grip shows number 1 you are in fact in the second gear and so on.
As there is no information supplied at all on the gears would appreciate any advice as to whether this a cable adjustment or does one fiddle with the H and L screws down the bottom?

Maybe this will help...
http://www.webmountainbike.com/adrearder.html

Many thanks - it certainly will.
Ramon

The park tool site has some more specific instructions for those of us who are less mechanically inclined (i.e. me )

http://www.parktool.com/repair_help/FAQrindx.shtml

__________________
Peter
Cannondale Synapse Carbon Scalpel Cyclocross Ultra

Thanks Rek - that site is also helpful. The top cable adjuster solved my immediate problem. Whoopee can now use all the gears.

Incidentally can thorughly recommend the Dahon Boardwalk bike.

regards
Ramon