If you are using Unslung, before attempting to run ipkg, install or use a package, or update the list of packages, make sure that you have installed a hard drive or memory stick and "unslung" to it. Failure to do so may fill your NSLU2's internal Flash RAM with too much data and not allow it to reboot. To recover, follow the Recover From A Bad Flash process.
When using Unslung, look under /opt for the corresponding files for a particular package.
If you're having problems at this stage connecting with ipkg, see this troubleshooting FAQ.