Since the common user does not have permission to install programs (Since installing programs is an administrative activity) you need to start synaptic as an administrator. To do this you can:
- Start synaptic through the menu: System -> Administration -> Synaptic
- Press ALT+F2 and type: gksu synaptic (gksu is to start a program in gui as administrator)
If you run it like one would normally would, just typing synaptic, synaptic will run with the privileges that the running user has. Since the privileges of the normal user are not administrative level, synaptic throughs that security message. At that level, you can only see the programs you can install and information related to them, but you can not install from the lack of administrative privileges.
NOTE - If you edit the main menu and go to the synaptic option of the menu, you will see it also starts with gksu. That is why in the menu it works like it should.