To use models in your project is so easy, check the steps:

• In your project go to file/append or press shift + F1

• In the window that appear, go to directory where you unziped the vegetation pack, and click in "blendermarket_vegetationpack_1_0_X.blend", and go to "object", check this image:

•  Check this catalog to view model you want
• And select the object with same name in the blender
• The pack contain groups too, do the same procedure but in the area of ​​group

If you like, check this video in this link, to view how to append models.