To search your entire library, make sure All Documents is selected in the left hand menu. If you want to search only those references in a particular group, select that group.
At the top right corner is a search box which can be used to find references within your own library.
Mendeley Desktop continues to be available for existing users but is no longer available to download and is no longer under development. For guidance on using the new Mendeley Reference Manager, Mendeley Cite and Mendeley Web, see our Mendeley guide.