When you will hit enter after putting the trendmicro/bestbuy url either your program will start downloading automatically or it will show you some options like save , run and cancel on the bottom right of the screen. It actually depends on your browser settings.
If it is not downloading automatically , just click on run or save on the bottom side.
seems like you program is downloaded ? then just double click on the program to install (run) it.
Now it will ask for your activation code. Follow the on screen instructions coming from the trendmicro installer file.
Finally your program is installed and ready for its first scan.