If stereo sound is desired from the ME-25, we recommend connecting the PHONES output on the ME-25 to two inputs on your mixer using a "Stereo Insert" cable. You can find these cables available at most all music gear retailers.
It is 1/4" stereo on one end, with two mono (L/R or tip/ring) 1/4" cables on the opposite end. Connecting to the PHONES output activates the guitar amp simulator in the ME-25, which can be desirable when connecting to a PA system.
The next alternative is to simply connect the L/MONO 1/4" output on the ME-25 to the mixer using a standard mono guitar cable. If the mixer is going to be further than 25ft or so from the ME-25, the use of a DI [direct] box is recommend to keep the signal strong with low noise.
You can see further connection details on page 2 in the ME-25 owners manual
ME-25 Owner's Manual