Microsoft Excel does not support complex numbers by default. You need to enable the Analysis Toolpak in order to create complex numbers -- without that, anything you enter in complex form is just considered to be a text string.
I see that writetable() from MATLAB drops complex components as it writes, so it is not possible to just use the work-around of writetable() to get complex data into a form that can be read back.
I happen to be using Excel for Mac 2011. It turns out that the Analysis add-on is not available for that; they refer me to the third-party xlstat package, which would be a minimum of $US295 dollars per year (I do not qualify as academic.) And that is minimum -- there are premium add-ons and it is not clear which of those would be needed to support complex numbers.
I would suggest to you that you simply make the complex component a third column in the xlsx file, and put the two columns back together again after reading in the data, by using
U = complex(a(:,2),a(:,3))