Interpolation

I have a problem involving interpolation. I have two columns of data that vary depending on input parameters. I would like to find an iterpolated value within the second column based on data from the first column. I want to keep from adding a trend line, and would rather have the computer take an interpolation value I select, search the 50+ data points in the first column, find values that are directly above and below, interpolate, and return the interpolated value from the second column. It is not a problem to have the fist column in ascending order, but I don't want to interpolate by trend line, rather linearly between the closest greater than and less than points.

I have search the internet and read something about an Interpol() function, but can not find it in excel as a function or an add-in. I could only find one page about it and could not get access.

Any assistance you can give me will be greatly appreciated. Thanks.