The waste ink absorber is not a user replacable part, so if you wantt to clean it, theen you have to do it at your own risk. You can follow the steps from the link below to remove and clean the absorber, after that follow the steps below to reset the ink absorber, please remember you have to follow tthe same steps more than once to reset the counter.
PROCEDUREI. Enter Service Mode:
II.Execute Waste Ink Counter Reset Operation:
When you turn on the printer,waste ink absorber full error is cleared and the printer work normally.
Original source: http://ij-forum.blogspot.com/2010/01/reset-waste-ink-counter-canon-pixma.html#ixzz3yHiLDtWy