Probably the search service is not installed on your site. In order to add search service you need to install EPiServer.Search package on your site. Here is more details.

Edit: Sorry, I just realised the question was about EPiServer 7, not EPiServer 7.5. 

As @Sergii says, the internal search requires you to install EPiServer Search.

If you downloaded EPiServer 7 and ran setup.exe, you should be given the choice to install the EPiServer Search service.
(Otherwise you can run EPiServerSearch.msi from the EPiServer 7 zip file manually)

You should also be given this option via Deployment Center. Have a look at this blog post for details:
http://sveinaandahl.blogspot.no/2013/06/how-to-install-episerver-search-for.html

Once installed, log back into your site and go to Admin > Config > Search configuration.
Make sure Blocks, Files and Pages are checked.