step 1 dynamic loader

sudo gedit /etc/ld.so.conf

add the <path>/build to the end of the file

sudo ldconfig

ldconfig creates the necessary links and cache to the most recent shared libraries found in the directories specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories,/lib and /usr/lib

step 2 debug a "waf" built ns-3 application

In qtcreator, first set click Projects on the left and then under Targets and Desktop select Run.

Under Run Configuration, set Executable to <path>/build/XXX/ns3-cppname-debug

Use Debug->Start Debugging->Start Debugging.