What he said.
That probably means it is a PC made tif, or that it is greyscale or cmyk and not rgb. Open in photoshop on your mac, make sure it is rgb and do a save as. BTW, I didn't know you could save transparent layers as a tif. You may have to save it as a PSD file for that, which is fine, FCP likes those too.
good luck!
Kevin
"A problem can never be solved by the same consciousness that created it"
Einstein