Manual:$wgNamespacesToBeSearchedDefault

Details
The variable holds an array of namespaces, numbered from -1 (Special:) and not including -2 (direct-linked media) indicating which namespaces are enabled for searching by default. The array is indexed by the numeric namespace ID, and each index holds either 0 (don't search) or 1 (search this namespace).

Note that changing this value only affects newly created and anonymous users - it does not change the settings for existing users.

Default value (1.5.0 and above)
Change "false" to "true" to set the namespaces new users will search.

Changing options for existing users
The problem, unfortunately, applies even to namespaces created after a user was created/has edited his/her preferences. The problem is that method  in   does not fall back to default user options if a particular option does not exist in that user's preferences. This is quite annoying if you use namespaces to categorize article types since there is no easy way to update preferences of all users. The following implementation of  solves the problem - it uses default values for options which the user did not touch yet:



Default value (prior to 1.5.0)
The NS_ constants weren't used (possibly because they weren't defined) so numeric values were used instead:

Default value (prior to 1.4.5)
Prior to 1.4.5, namespaces 9 and 11 were also searched.

Another method
This is an example of what to edit if nothing else works.