Excel has functions for manipulating complex numbers. A Google search for "excel complex numbers" gave me this as the first hit:
http://mysite.avemaria.edu/jcdaly/Tutorials/ComplexExcel/ComplexNos.html
To see if you have these functions, enter the formula "=complex(2,3)" in a cell. You should get "2+3i". If you get an error, you may have to install the Analysis Toolpak. I remember having to do that in an older version of Excel. My current version (2011 for Mac OS) apparently has it built in.
You have to do all arithmetic using functions. For example, if you have 2+3i in cell A1, you can't use the formula "=2*A1" to multiply it by 2. You have to use "=improduct(2,A1)".