help with I2C expander

I need to find a routine to use a I2C expander to read a 4 x 4 keypad. I want to produce an interrupt when a key is pressed and read the key being pressed. The expander will be a MCP 23008.

Well, you got your coding cut out for you then. Set up four pins as outputs, and four as inputs with external pullup resistors.
Set one output pin low, see if you get an interrupt from one of the inputs being low indicating a key was pressed, if so then read all four inputs to see which one it was. The combination of knowing which output was low and which read low lets you know which key was pressed.
Set the output high, and repeat for the next output pin.

Have to deal with the chip’s registers to get the states set up for inputs & outputs, complicates things a bit.
Have you checked for libraries that might do this already?