Dave: The 18X can only be an i2c master device.
The i2cSlave command, used on the 18X, is used to tell it the slave's address and speed that it (as master) will talk to its slave/s.
Only the x1s (and x2s, when they are released) are able to perform the slave role. These are configured using the Hi2cSetup command, which you will see can't be used on the 18x.
-Peter