Where do I find meta keywords in the database?
When saving new keywords we get an error saying "unable to load sitemap data".
Odd. What version of C5 is installed?
I need to do this manually for a client. I wonder what db table it's stored in.
Using the latest version. 188.8.131.52 I think.
If yes, is it writeable?
I am really unsure why it would be trying to get keywords in a sitemap.
Those are attributes tied to the collection object. To access them directly, take a look at the AttributeKeys table to find the id for the meta_description attribute (for me it's id=2). Then take a look at the CollectionAttributeValues table and map the collection ID for the page your wanting to update the value for with the akID (the one for the meta_description) and find the avID (attribute value ID). It looks like the actual content is then stored in the atDefault table.
I'd still recommend tracking down the issue that you're running into instead of database-diving and changing things there since there are most likely some other joins that I'm leaving out here. But hopefully this will get you on the path to put out the fire and then you can track down the actual issue.