Make sure you are logged in to the website. If you have not registered, you must do so.
Go to the T-Map map application There should already be a tamarisk layer on the map (it may take a while to draw because there is a lot of data). Make sure tamarisk is highlighted in the legend. It should be by default.
Click the download tool in the upper right - it's the one with the green down arrow.
Draw a box around the area on the map you want tamarisk data for.
Follow the steps to further refine what data you would like to download. Use the help for each page by clicking the 'help link in the upper right hand corner of each page.