Open the project you want to import assets into. Choose Assets > Import Package > plus the name of the package you want to import, and the Import Unity Package dialog box displays, with all the items in the package pre-checked, ready to install.

Go to the Unity Asset Store. Search for the asset you are looking for using the Search for assets box. Alternatively, browse the categories using the drop down menus at the top of the page. Once you have found the asset you are looking to purchase, select the asset to view the product details and click Add to Cart.