do a search and you will find a few threads here.

simple passive DI boxes backwards and simple resistor adapters can do the job with ease.

Is this a recorded track of the keyboard that you want to play back through the amp? Is this a "live" take where you want to play through the amp while you are recording? You may be able to plug the keys right into the amp without too much trouble, especially if you're attempting to get some overdrive effects. Otherwise, you should probably be using a PASSIVE DI box (Radial and ProCo make such items, as well as Whirlwind) to drop the level and match the loads better. Does your mixer have a selector between "+4/-10dBm" levels? Set it at -10 to minimize the "over-overdriving" of the amps input...that may be enough to get you there....

what are resistor adaptors? i understand about the passive di but not about the adaptors?

simple adapter units or cables with resitive pads built in

a variety of combination are available from different makers and they provive the building blocks to make all kinds of pad/adapters for mic / line and balanced / unbalanced.

these can be expensive and adjustable as per the shure xl to xl pad

or be cheap and DIY with a single pad amount

I like to use eddystone boxes with multiple connectors and a rotary switch to get the combinations I want.