Getting the logo into 2D IGES was the key for me. Once in this format, while in a drawing or format file, you can select Create>Symbol>Definition>Define. This will bring up the Symbol dialog where you can specify a bunch of things about the symbol you are creating/importing (IE text, scale, size, placement origin, etc). I suggest doing a few trials with it before you do anything with your final logo. Don't forget to use the Write command in the Symbol menu to save the symbol (in the current directory) for use later.
To retrieve an existing symbol, use Create>Symbol>Instance and select Retrieve in the resulting dialog. If you have the config option PRO_SYMBOL_DIR set, it will default to the directory you have already set up.
Keep in mind that symbols in ProE are like 'cells' or other placeable graphics (clipart) - once they are properly defined, they are very useful for repeating the same graphic over and over (charts, graphs, or whatever silly sales guys want on prints).
Recneps