Hi Stephen, your question reminds us of that old joke about the guy who dropped his keys a block back, but when asked why he is looking over here, he answers, because the light is better.

Sorry, your question is really about operating your Cakewalk A-Pro 500 (?) (whatever that is...) However, we doubt very much that an Expression pedal can send Program Change messages. That is not how MIDI works, in general.

Expression is one of several MIDI messages classified as channel Control Change messages... cc011
Program Change messages are an entirely separate message type within MIDI.

Besides a CONTROL CHANGE device like a Foot Pedal sends values between 0-127, for the Program Change message you need to send both Bank Select and Program Change values. You'd never be able to select an exact number like 045, on demand... Your feet are just not that accurate, sorry. That's why an Expression pedal is not available to send Program Change messages (in most devices). We don't know anything about your controller (sorry).

Anyway if you have any questions concerning your Yamaha MX we'd be happy to help you..

You can select PERFORMANCES on the MX by sending the following commands on the BASIC RECEIVE CHANNEL:
MSB = 063
LSB = 080
PC = 0-127

You can select individual VOICES on the MX by sending the following commands on the individual Receive Channel number that matches the PART number:
Preset 1: MSB 063/LSB 000 _ PC 001-127
Preset 2: MSB 063/LSB 001
Preset 3: MSB 063/LSB 002
Preset 4: MSB 063/LSB 003
Preset 5: MSB 063/LSB 004
Preset 6: MSB 063/LSB 005
Preset 7: MSB 063/LSB 006
Preset 8: MSB 063/LSB 007
Preset Drums: MSB 063/LSB 032 _ PC 000-063
User 1: MSB 063/LSB 008 _ PC 000-127
User Drums: MSB 063/LSB 040 _ PC 000-007
GM normal: MSB 000/LSB 000 _ PC 000-127
GM drum: MSB 127/LSB 000 _ PC 000

For what you can send, and how you can send it from your Cakewalk Controller you'll need to contact them.