I have seen customers doing mistake when connecting a negative signal to PSoC. Since, PSoC works only on positive supply, so negative signal can not be connected directly to any of the digital or analog block.
There is a very nice blog written by Ganesh Raaja on how to measure Negative Voltage using PSoC1, it can be accessed using this link: http://www.cypress.com/?rID=50815&cache=0
Happy working with PSoC.
I did a design idea on this in Electronic Design that will allow you to calculate the resistors needed to shift the voltage using Excel