Might as well give this old question an answer.

cgminer 3.7.2 (which was obsolete even when this question was asked) required scrypt to be enabled as a compile-time option with ./configure --enable-scrypt. The asker evidently did not enable it, which is why it does not work.

It appears that current versions of cgminer have removed scrypt support altogether.

As Hồ Tuấn Kiệt points out, a Raspberry Pi would be hopelessly underpowered for CPU mining of any kind, so this would be a really ineffective idea even if it worked.

The asker probably got this idea because there is lots of discussion on the Internet about using the Raspberry Pi for mining. However, most people use it in a very different way: not to do the mining itself, but as a controller for ASIC mining devices (typically for Bitcoin and other SHA256 coins, not scrypt). It is well suited to the latter task, since controlling ASIC miners requires very little in the way of CPU performance; just the ability to run a minimal operating system with network access, and some simple software to fetch work from the network and feed it to the ASICs. The Raspberry Pi is certainly capable of that task, and it is cheap and uses little electricity of its own. But it isn't suitable for actually mining with its own CPU or GPU.