Instructions to use RebuildWalls: 

1. In a Revit Project, select an imported DWG file.
2. Hide layers not needed.
3. Select the DWG and click on Walls and Lines Button from the Rebuild Ribbon Panel.
4. Choose options from the Rebuild Walls window (Single or Double Lines, Wall Type, Level and Model Line Options). 
5. Click OK.

The old way (using Excel) is still available. Here are the steps: 

1. Open a plan from AutoCAD (2012 or 2013).
2. Isolate the lines and arcs used to create walls in Revit.
3. Lines must be separated so their length is equal or almost equal.
4. Use the AETEXT command (or DATAEXTRACTION.
5. Create a .dxe file.
6. Click Next and select only the lines used to create the walls.
7. Click Next to the data file creation and save an .xls file.
8. From Revit, click on Walls from Revit, get the .xls file and select the units, line method, wall type (width equal or near the distance between the double lines), Level and Model Line Option.
9. Click Ok.

The installer that ran when you downloaded this app/plug-in from Autodesk App Store will start installing the app/plug-in. OR, simply double-click the downloaded installer to install the app/plugin.

You may need to restart the Autodesk product to activate the app/plug-in.

To uninstall this plug-in, exit the Autodesk product if you are currently running it, simply rerun the installer, and select the "Uninstall" button. OR, click Control Panel > Programs > Programs and Features (Windows 7/8/8.1/10) and uninstall as you would any other application from your system.

More info's can also be found here:
http://www.emanuelfavreau.com/rebuild2infos.php

Notes for the 'Excel' way:

• AutoCAD units should not contain both feet and inches together, it will not work if an Excel cell contain for example a value like 25'-2 3/4", units must be either in feet, inches, meters or millimeters ("." is accepted, not ",").
• Select only lines and arcs when creating the Excel file, no text, no dimensions, etc.
• The double line method is different from the single line method. It will calculate a center line between two lines. To make this work, the AutoCAD lines length must be equal or almost equal.
• Also, for better results, try to choose a wall with about the same width as in the AutoCAD drawing.
• Another important thing is that polylines must be converted into lines and arcs before creating the Excel file.

More info’s can also be found here:

http://www.emanuelfavreau.com/rebuild2infos.php