How do I wire up this relay?
Ignore pin 4, that's the normally closed contact. It's that simple. Note that the diagram is looking at the bottom of the part, not from the top through it. 

1 and 3 are the switch that closes when the coil is energized, it doesn't matter which is which, and they go to the keyboard. 

2 and 5 are the coil.. again it doesn't matter which is which. 

Test it out before hooking it up to the keyboard- you should hear the relay clacking as it operates with the "tumbler switch". 

When you do hook it up, keep the connections neat, shorts kill parts.