Simple -- select the cell (or range of cells) to which you want to apply the format. Right-click on the selected cell(s) and select "Format cells" from the context menu. Click the "Number" tab, and then select "Scientific" from the list of number format options...
format the cell, and under category choose CUSTOM... and in the box choose ##0.00E+0 and you should be all set.
crust... there is no format box under NUMBER, it's under CUSTOM... at least in the version of office that I have... this may differ in other versions...
Oops -- I misread that one! Of course, it did seem like too simple a question.... and yes, the "custom" method posted is the answer for powers-of-three notation...