It is quite a job getting cpufreq working on the original DSL, because it uses an old kernel (2.4 series) to save space, hence the lack of ACPI support.
If you like the DSL distro but need to support a wider range of things, have a look at their alternate distro called
Damn Small Linux Not. It runs a 2.6 series kernel, so cpufreq will be a lot easier to get going. It doesn't fit in 50Mb, but has a similar look and feel, and is still lean on resources compared to most distros.