Did you install noip through yum? The yum RPM package should install the noip service script in /etc/init.d/ for you. Run the following commands as root to install and setup noip through yum.
yum install noip
service noip start
To configure the noip service to automatically run at startup, run the following command as root.
If you really want to know the nitty gritty details of adding a service, here's a good tutorial on manually adding a service.
To answer your second question, the --levels parameter tells which run levels a an operation should run in. Linux has seven run levels by default. Each run level can start different set of services. For example, Runlevel 1 will put the system into Single User Mode, which only starts most basic services and puts you into a root shell. Runlevel 3 is multi-user mode with no GUI. Runlevel 5 is similar to Runlevel 3 except it also includes the (GUI). Runlevel 6 is another special mode that will reboot the system.
More detail about run levels can be found at Wikipedia