Bottom line, call Delta.

I've flown to/from Nairobi on several occasions, nearly every time upgrading my ticket from full fare economy to biz class. Every time I've struck out trying to upgrade via the website, but was able to find an upgrade by calling Delta. Previously, it took a long time to secure the upgrade because Delta and KLM were not synced up. However, now they have apparently "fixed" their system and they were able to process the upgrade for all the flights in one step.

There are a several routes into Nairobi, the most popular being from Paris and Amsterdam. Personally, I prefer the Amsterdam route.

The other option to consider is to purchase a full fare economy class ticket, and then upgrade to biz class for 50k miles. Not a free ticket, but it would be an easier award to get.