From: Harlow Vaniva
They are in your build menu, the last tool you can select (like a cube or sphere, but look past all the shapes and you'll see a tree). At least I think that's what you're talking about.
This method only gives you a random plant or Grass choice. You have no control over what you get, so this tool is rather worthless.
As Osprey stated, all the pre-existing Linden plant types are in the Library section of your inventory, under Objects > Trees, plants and grasses. Find what you want there, and drag them to the ground to rez them.
You must be on a parcel that you own, or that you have a specific group role in that allows placing Linden Plants, or this will not work. You can't plant a tree on land owned by someone else, or owned by a group that you have no group role for tree planting in.
The Linden plants are nice, but have their limits.
There is a limited selection of them. Not all plant types that you may want or need exist, and most of them are more suitable to a tropical island or a coniferous forest or a sea bottom.
They react to the in-world "wind", (if your client doesn't have this disabled, and after version 1.23.4 of the client you have to manually enable this feature to see it). But they react too much when first rezzed, looking like a Hurricane is in progress. They do settle down eventually though, and sway gently.
Linden Plants react strangely with distance and with lowered graphics settings. Their appearance changes noticably as you get further away, and with a long draw distance this may mean they only look good when you are very close to them, and on maxxed graphics settings.
Their biggest advantage is that they take only one prim to place one, where virtually any other plant made of prims takes three or more prims per plant.
Prim plants, on the other hand, can also be designed to sway in the wind (though they usually are not), and can be any type imaginable.