Go >File >Import from another Catalog and click on the catalog transferred from your laptop. This will merge the catalog from your laptop with your catalog currently open.
In Lr3 the menu is >File >Import from catalog.
Before you start: backup both catalogs to a safe location and read up in the Help menu on <import from catalog>. There are several options and you need to choose wisely.
If you copied your catalog to laptop, worked there, and didn't touch the
desktop copy all this time — there's no need to waste time on
exporting/importing. Just replace the older catalog with the newer one.