for
RichMemo you want to grab the sources from
here. It can be done easily by using SVN client (such as tortoise svn or command line).
As you can see (by the link above) there's file "richmemopackage.lpk" among these sources. You need to start Lazarus then go to "Packages" menu, select "Open Package" and then select this file.
The package dialog will open. There you'll need to press "Install" to install the package. It will require Lazarus to be rebuilt and restart.
As a result you should see RichMemo available in "Common Controls"