If you want to be able to connect to eDonkey servers to look for sources in the eDonkey network you must set this variable to true.
If you yet have a HighID everything is fine.
If you get a LowID current setting of 'enable_servers' isn't your problem (it should be true as you couldn't be connected otherwise).
If you cannot connect to eDonkey servers at all (neither LowID nor HighID) have a look at your current setting of 'enable_servers', it's probably 'false'.
Don't confuse this with enable_server which (IMHO) is a typo/doesn't exist (at least in mldonkey-2.5.28).
_It's not, MLdonkey can also work as a donkey server. That feature hasn't been worked on a very long time however (last time the Changelog mentions it in prerelease 1.99, around 2 years and a half ago), and also the implementation was never meant to scale to the same degree as the official server software. So it's probably wiser to leave that other feature disabled._
> set enable_servers true