First open the elevation where you want to add dimensions.

In the "Elevation" option on the top right of the screen, click on the drop down next to it and you will see an option "Elevation 1 (Wall)". Click on that to view the elevation.

In the toolbar section, go to the "Annotate" tab and click on the type of dimensioning you want to add out of Align/ Angle/ XY/ Radial.

To understand the different types of dimensioning available check this article,

http://help.infurnia.com/en/articles/8262799-what-are-the-different-types-of-dimensioning-options-you-can-use

When you select any option, the design panel on the left of your screen will show the settings of how the dimensioning needs to be done. Check this article for details,

http://help.infurnia.com/en/articles/8281875-how-to-change-the-dimension-settings

Click on the start and end point of the distance you want to indicate and the dimension will be added. Place it as per your requirement.