Input level from wireless system

I run a Sennheiser D1 wireless system in front of my amps. In the options you can only choose between mic and line level outputs to the amp. How do I get it to instrument level? I have emailed Sennhieser twice and the customer service in Australia is very poor and I got told to read the manual. I have about fifty times. It doesn't say anywhere what level to run when using a guitar? I can go up in 1db increments from 0db in the line level section of the manual but can't go down. What exactly is instrument level in db's?

http://www.ovnilab.com/articles/linelevel.shtml
http://shure.custhelp.com/app/answers/detail/a_id/758/~/mic-and-line-level----what-do-they-mean%3F

I'd go with mic level. That is generally lower than either instrument or line. Then add gain as necessary.

Frankly I'm a bit stumped with your transmitter/receiver, as they are usually set up to give no gain or attenuation. Are you using a general purpose receiver that was somehow retasked for guitar? A guitar receiver shouldn't be terribly adjustable.

It's the new digital unit from Sennheiser it has many options and the receiver is able to be used with mics. So it has a de-esser, eq and a few other features I don't need.