I can't seem to find the pull down menu that I can customize with any vegetable. My vegetable selector won't let me type over anything.

I am not sure where the problem, but I have some ideas.

The first thing to check is to see if your plant info window is hidden.
You will see a radio button on the top middle of the screen that says "Plant Info"

If it is a grey circle with a white filling, then it is turned off and this will fix your problem.

If it is a grey circle with a green filling, then it should be somewhere on your screen. Check to see if you may have moved it to the corner or nearly off the screen to get it out of your way. If you move it back onto the screen this is where you want it to be.

The pulldown you are looking for is in the Plant Info window.
Unlike normal pulldowns, this is special and you can actually select the text and type over it. This will create a new plant variety.
If you look at some of the demos, they actually show this.
http://www.plangarden.com/manual/Manage_Vegetables.htm

If you are still having problems, please let me know. I am more than happy to help you figure this out and get your garden going.