CPDN applications are 32bit. Some 64bit Linux distributions do not have 32bit compatibility libraries installed by default. In order to run cpdn on a 64bit Linux PC without errors, you may have to install these compatibility libraries.
See this entry in the BOINC wiki for more information or this one for a more recent iteration of the instructions.
Edit...The above links work for some versions of Ubuntu/Debian and Fedora. The 32 bit compatibility libraries may be named differently on later distributions of these operating systems.
Edit...you can run the command "ldd" from a terminal window on the cpdn executable programs which should give you the dependencies that need to be installed.
Edit...Additionally, in order to see the graphic of the globe, you may also need to install some other 32 bit packages. If you have trouble displaying the globe graphic, restart boincmgr or run_manager in a terminal window if you didn't already start it that way. Then try to display the globe graphic. If it doesn't come up, note the errors in the terminal window about missing libraries. Install the 32bit version of the missing library, then try again. It may give another error about another library, install the 32bit version of that library...etc. libpng and libjpeg are at least two of the problem libraries. Depending on your linux distribution, some of these 32bit packages may be installed by default, or as a dependency of a different 32bit package you might have installed, so you only need to do this troubleshooting if the globe graphic doesn't display.