"I have never come across software for landscape design that was worth anything."
I have to agree with this statement. I had tried some Windows based software and it was more trouble than it was worth. Either the plants couldn't be adjusted to the right size or the plants I was look for were not in the program.
I use graph paper and have looked in many gardening magazines and books for ideas, then measure, plot on the graph paper and go!
Keeping a graph of what is planted and where to scale has been helpful as we add things to the gardens or try to remember what is coming up where because we use a lot of bulbs. Good luck!