The big search lesson to take from the answer to this challenge is this: Don't assume too much about what you think the answer will be. Your initial assumptions might be very wrong, and you'd waste a lot of time trying to prove something that just isn't true.
Here's the story...
Background: Cranberries (Vaccinium macrocarpon), are native to New England and a few other places in North America, growing in acidic bogs. Many members of the heath family, such as blueberries (Vaccinium spp.) and azaleas (Rhododendron spp.), also grow well in acid, peat soils.
The cranberry plant is a very long-lived perennial less than eight inches high with trailing, thin stems with small, opposite, evergreen leaves. Cranberry flowers appear around the Fourth of July; these are white to light pink, downward-pointing, bell-shaped, axillary flowers. The name cranberry is a modification of the colonial name "crane berry," because the drooping flower looked like the neck and head of the sand crane, which was often seen eating the fruits.
Cranberry sauce, as we typically think of it, is a cooked, heavily sweetened concoction that's frequently augmented with citrus. Of course, any citrus would have been impossible to have in 17th century Massachussets. An even larger problems is that there was effectively little sugar in 1621 America. (Certainly not in sufficient quantities to make cranberry sauce!) The settlers hadn't had time to make maple syrup or sugar, and honeybees were still years in the future.
But how do we search for this kind of information?
My first search was [ cranberries 1621 ] which gives a number of search results, the most interesting of which is the "The Truth About Thanksgiving" from the Planet Blacksburg (VA) news site that quotes Daniel Thorp (colonial history prof at Virginia Tech)
After looking through lots of deadend links (research is a slow process!) I finally decided to look for the original letters describing the Thanksgiving holiday in 1621 to see what I could find. From the Daniel Thorp article I found that the letter was written by Edward Winslow, making the obvious search
[ Edward Winslow 1621 ] lead me directly to a transcription (and handy translation into modern speech): http://www.pilgrimhall.org/1stthnks.htm
I quote from their site (in modern spelling):
"...our harvest being gotten in, our governor sent four men on fowling, that so we might after a special manner rejoice together, after we had gathered the fruits of our labors; they four in one day killed as much fowl, as with a little help beside, served the Company almost a week, at which time amongst other Recreations, we exercised our Arms, many of the Indians coming amongst us, and amongst the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five Deer, which they brought to the Plantation and bestowed on our Governor, and upon the Captain and others. And although it be not always so plentiful, as it was at this time with us, yet by the goodness of God, we are so far from want, that we often wish you partakers of our plenty."
From there, I found that William Bradford had also written about the First Thanksgiving. The obvious query:
[ William Bradford Plimouth Plantation ] leads to his writings on Thanksgiving.
And that's about it for the historical record. So, were cranberries on the menu, and if so, in what form?
There certainly was a tradition of stewed fruits in 17th century English cooking, so it's probable that some kind of fruit cooked compote was served, but it's not actually in the record. So the truth is... it's all speculation!
Most probably, the Indians (the local Wampanoag) brought along some of their own supplies. Ninety people is, after all, a pretty big crowd to bring to Thanksgiving table. If so, then they most likely brought along cranberries in the form of pemmican, a very calorically-dense survival and travel food. It's made by mixing dried berries with dried deer meat and melted fat to form easy-to-carry slabs. Think of ground up beef jerky mixed with a bit of bacon grease and dried cranberries, and you've got the idea.
Not my solution, but very nice.