.pas is the source code, *.dcu is delphi compiled unit and *.dpk is
delphi package source code.
some component only gives a pas file and many dcu files, then you should have the same version of delphi as the author. open componet
install component and install the pas file,that is ok.
if you get all the source code, do as before but you may have different version delphi.
the pas file may have a name of ***reg.pas ,*** is the component name.
if you got a package with dpk file ,just open it from file menu then compile.
if you got dpl(delphi package lib) or bpl(borland package lib) just install this package.
if you got dpc file , do as dpl file ,it will lead you to an install interface. Famouse AHM97 is supplied in this kind.