Hi,
The sockets on the back of your keyboard are:
L/L+R= (L):The Left Stereo channel to be combined with the (R):Right stereo channel,
(to be used with a stereo AMP).

"OR" L+R:Left & Right channels mixed together "in mono" to be used with a mono AMP

You need 1/4" Mono Jack plugs one end to a stereo jack the other end. You can use adaptors as dlstarry suggested. I hope this helps.

Dave