See page 142 of the manual for instructions.
The WK-7600 will playback either Type 0 or Type 1 Standard MIDI Files (SMF) directly from the SD RAM memory card. The files must have a ".mid" file extension and must be in the "MUSICDAT" folder on the memory card. A computer can be used to copy files to the card.
If your Yamaha MIDI files were voiced to use the Yamaha PSR GM or XG voice set, then they will most likely playback correctly on the WK-7600 using its GM voice set, but if they were voiced to use the Yamaha proprietary voice set, then they will need to be revoiced to either the WK-7600's proprietary voice set or its GM voice set in order to sound correctly. GM SFX sounds (gunshot, wind, rain, applause, etc) will not be supported and EFX (reverb, chorus, pan etc) may or may not be supported, depending on how they were recorded.
As an alternative, you can play MIDI files to the WK-7600 directly from a computer based DAW or software MIDI file player via the USB cable instead of loading them to an SD RAM card. This would be a particular advantage if the files need to be revoiced as stated above.