Q: can i download office mac 2008 using the product key only

If yours is a newest model 12-inch MacBook retina, the version microsoft office 2008 likely is too old.

https://duckduckgo.com/?q=microsoft+office+2008+for+Yosemite+OS+X+10.10&t=ffsb

Please specify what version and model build your MacBook product is.

You should ask Microsoft that question. My guess is that it would only be available on DVD.

Office 2008 runs in Yosemite and there should be no reason why it wouldn't work on a 12" MacBook Early 2015.

Except some of the installers are Rosetta only, which requires Mac OS X 10.6.8 or earlier.

Not true, see screen shot from my 12" MacBook Early 2015, running Yosemite, the installer works just fine, but I'm not going to do the full install just to prove a point. There were many reports about this Rosetta thing back when Lion was introduced and none of them were verifiable.

However, I don't believe that Microsoft ever offered Office 2008 as an on-line download, but that's something the OP should pursue with Microsoft.