You will first have to install a Subversion client. The KFTPGrabber is part of KDE Extragear, so you can get it out from their repository. Use the following commands to do a SVN checkout:
svn co -N svn://anonsvn.kde.org/home/kde/trunk/extragear/network
cd network
svn up kftpgrabber
svn co svn://anonsvn.kde.org/home/kde/branches/KDE/3.5/kde-common/admin
make -f Makefile.cvs
./configure --prefix=`kde-config --prefix` --enable-debug=full
make
su -c "make install"
This will install the latest version of KFTPGrabber.