The Add Folder to Library ... function is available via the File menu; if necessary, enable the menu bar by pressing Cttl-B (or select Show Menu Bar from the "mini menu" accessed using the little black/white square icon/dropdown at the top right of the iTunes window).
However, you shouldn't really be using this function to migrate your library from an old computer to the new one - you can (and should) move your complete library so that you not only retain access to your media but also to playlists, playcounts, ratings, etc. The following resources are relevant:
- How to move your iTunes library to a new computer - Apple Support is the "official" guidance on this topic
- Turningtest2's user tip on Backup your iTunes for Windows library with SyncToy describes a better / more robust method to copy the iTunes library from your old PC (typically to an external / USB hard drive) from which you can then copy it to your new PC
- Tt2's user tip on Make a split library portable provides a lot more detail on the operations that may be needed to bring your library into the correct layout to make it fully portable between systems - the "official" guidance referred to above correctly references iTunes Consolidate Library function as part of this, but in some cases - particularly if you're using a non-standard way to store your media - additional info my be useful.
Once you've completed the migration process, make sure you create and maintain a complete backup of your library (I'd definitely recommend using SyncToy as described in the second reference above). With this process in place moving your library to a new PC becomes a simple and issue-free process.
