(from memory, assuming you're using a Linear trend)
There are 2 ways I've used before, both using the slope and intercept values of the trend:
1) Use the SLOPE and INTERCEPT functions on the data to get those values;
2) Select the trend line and open its options. You have an option to display the equation of the line, in the form y=mx+c. m is the slope, c is the intercept.
When you have the slope and intercept, you can use them to calculate the value at any point on the trend line, using y=mx+c again. Then you can subtract those values from each y data value to get the error.