Dear Xinyue,

I am not aware which sample was selected for inclusion in nilearn. Maybe better to reach out to nilearn developpers. Latest documentation describing the ADHD200 preprocessed sample is available here:
http://preprocessed-connectomes-project....

and the following paper (currently under review):
http://biorxiv.org/content/early/2016/01...

Also you may want to register to the forum of the preprocessed connectome project, which regroups initiatives such as ADHD200 preprocessed and ABIDE preprocessed. 
https://groups.google.com/forum/#!forum/...

Best,

Pierre