Run opkg update first, then opkg install nodejs to install. Unfortunatelly it is still using an old version (0.10.5) of Node, there's no npm and not all packages can be installed (at least the ones that need to be compiled since the Onion processor is MIPS architecture). We know that Onion team is working in providing adequate Nodejs support together with the libraries and SDKs to interface with the GPIO pins. However, since the roadmap has not been published yet, we don't know when we can expect these features to be available.