Trayfreq (pronounced trayfreek) is a GTK+ application released under GPL that lets you select your CPU's governor or frequency from a tray icon; therefore, trayfreq only depends GTK+ and something that shows tray icons.
CPU scaling requires you to have a kernel with the ability already built in or have an appropriate driver module loaded. If the former case represents you, then you do not have to worry. If the latter case represents you, follow the following steps:
Most modern computers use the module acpi-cpufreq. Other options include the p4-clockmod, powernow-k6, powernow-k7, powernow-k8, and speedstep-centrino drivers.
To load the driver:
# modprobe acpi-cpufreq