Hello,
I am trying to install the my network but i failed.
It appeared in the Admin CP ,and I clicked the install also but the changes are not being shown
in the forum ,I am using MyBB 1.6.4
Please Help..

Does it say it is installed properly in the plugins list? It should say Activate/Uninstall next to it.

The MyNetwork Overview appears in the ACP index as soon as you log in, in the menu on the left. You have to install the features you want to appear on your site.

I installed it after it appeared in the list.
Now it shows Deactivate /Uninstall next to it.

It is also appearing in the ACP but I can't see any installation option for features as you are saying.
MyNetwork overview -> My Modules(Only one module mentioned named 'core') -> clicking core it says 'No Features were found for this Module...
'

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Also i am using page manager,akismet,profile comments and hello world plugin

^ the features need to be installed as separate plugins !

Where are these features appear

.r.is correct -hit the MyNetwork button at the top of the forum and on the right, choose Core or Profile System. The features are here to download.

If you want the profile System, you need to install the Profile System Core.