Hi marcov,
The Lazarus components are included in every platform package except the ARM packages. Each package contains all available language bindings. These are Lazarus (Free Pascal), Delphi, C++ Builder, Python and of course the native C interface.
The Lazarus packages to install the LCL components are located in rad\Lazarus\Packages. The Lazarus headers are in \rad\common. To access the basic API function you can use uSidexLib.pas/uTMLCore.pas. All other units are containing advanced wrappers to simplifiy the usage of the API.
You can find the documentation in the doc directory. On windows there is a .chm file and on Linux nad OS X you will find an html documentation.
I hope this is helpful.
Cheers,
Maik