importing music to ipod

You're on the right screen in iTunes (with the Sync Music checkbox). However, I'm not clear on what you want to accomplish. If the iPod currently has songs on it, that were synced to it from an old iTunes library, those songs will be erased if you set up syncing from a your current (different) iTunes library. iTunes replaces the iPod's existing content with content from your current iTunes library, and it is then associated with your current iTunes library for syncing going forward. When you use automatic syncing (not manual loading), the iPod can only be associated with one iTunes library at a time.

Therefore, IF there are songs on the iPod that are not in your current iTunes library (and you want to keep them), you need to off-load those songs first. iTunes syncing is typically in one direction only, from the iTunes library to the iPod. You cannot use iTunes to transfer song files from iPod to computer (except for songs purchased from the iTunes Store). However, there are third-party methods and utilities that can transfer from iPod to computer. If you do an Internet search on something like "ipod transfer music," you should get some useful links (including an article by CNET on the topic).

Once the iPod's song files are on your computer's drive, add them to your current iTunes library so that it has ALL of your songs. Confirm it's all there. Then, you can set up syncing of the iPod to your current iTunes library. Checkmark the box for Sync Music, on the iPod's Music settings screen in iTunes. This turns ON automatic syncing and associates the iPod to your current iTunes library. Any songs on the iPod that are not in your iTunes library will be lost; that should not be a problem anymore, since your iTunes library now has all of your songs. You can choose the option to sync Entire music library (or a selected portion of it). Click Apply. Going forward, iTunes keeps the iPod "in sync" with your music library (or selected portion of it).