Citadels and Engineering Complexes can be found on the overview, via d-scan, or by finding them on the scan overlay in space. Once you find a structure, save a bookmark with the name that the game suggests - if you edit the name of the bookmark, you may not get credit for discovering that structure. For some tips about locating structures see the Structure Spotting Guide.
Even though citadel structures have existed in New Eden for several months, they have been barely supported by CCP's APIs, causing problems for third party application developers. CCP have now added the ESI API which provides some information about structures that are public or that have you on the ACL. This is a good step forward and is perhaps all that some people need, however the ACL restriction means that it doesn't satisfactorily cover all use cases.
For example, if you have courier contracts to a structure you can't dock at, or you have assets in a structure you can no longer dock at, or you have wallet transactions from a structure that you cannot dock at, then there is no way to retrieve details about the location from CCP's APIs.
So until CCP develop a complete solution, I will continue to fund this crowd-sourcing project that populates this third-party Structure Name API available to all third party application developers in EVE.
There are several websites and apps that make use of the API that the structure bookmarks contribute to, the most prominent of which is jEveAssets.
This challenge will continue to run until CCP develop an API that meets all the requirements of players and third party app developers, whenever that may be.
When you bookmark a citadel or engineering complex, it saves the ID of the structure in the bookmark. By doing this we can link the name and location of the structure to its ID, which is important for third party developers. We ignore any bookmark that's not a structure, and we only request the "charBookmarks" scope so apart from your character name we can't get any other information about you.
You can disable this app's access to your bookmarks at any time over at CCP's Third Party Applications page.
No big deal - just visit your Profile and change your display name to whatever you want.
Let me know by sending an eve mail to "Messenger Of Truth".
If you want to increase the reward given for each new structure found (perhaps in a specific region or of a specific type), send an eve mail to "Messenger Of Truth".