Can someone help? I am having Trouble signing in from my Mac desktop?

What are you trying to sign in to?   It seems like you are online and using a username here in the forum.   If you have desktop software, you are not supposed to be "logging on" to use your software.  You are supposed to open up the program from your own hard drive.

We see people get confused and they keep logging in to online software which will require them to pay at the end.   You need to use the desktop software.   One way to tell if you are using the right software---your tool bar.   Online software has the toolbar in a black vertical column on the left side of the screen.   Desktop software has the tools up on the top of your TT screen.

Or do you mean you are having trouble opening up the program on your computer?   Did you download it, enter the license key,  and install updates?