Seems like you are missing a lot of the basics. You can't use a program without knowing how it works so you will have to do a little reading.

Start with the User's Guide. It's not that long and it gives you an introduction into all the basics.

To get a better understanding of how levels work you can read the "Levels explained" manual on the Dodecagon site. There are several more manuals you might find interesting.

You have a csv with dimensions. That should make it easy to resize boxes to create your beams and columns. Give them a wood texture. You can also import the Dimensional Lumber Library with 3D models for stick building. That library has pre-made beams you can resize.

The Roof generator is grayed out because you haven't selected a Room. The generator uses a room for the base dimensions of the roof. Read the help that comes with the plugin. In your case the room you draw for the roof is a simple rectangle. In the roof generator you can make sides invisible, set an angle, etc to create a roof. Since your roof is a simple slab of metal you can also use a resized box with a metal texture. Tilt it a little to get the angle you need.

By-the-way, you don't have to reboot after installing a plugin, just restart Sweet Home 3D.