To add a tab stop, make sure that New Tab Stop is selected. Click in the position box, and then you can either type the distance you wish the tab stop to be from the left edge of the column, or you can click on the ruler and the value will be calculated for you.
The alignment option lets you specify whether the tab stop should be left aligned, right aligned, centered, or decimal aligned. For decimal alignment, you can specify the character on which to align in the Decimal Char box. If you are writing a table of contents, or need tabs that will include a repeated character or string out to the next tab stop include that string in the Leader field. For example, you may want to have a . in that field for tab stops in a table of contents.
Once you have entered the specifications you desire, hit the <--- button. You will see the new tab stop listed. To edit that tab stop click on it, otherwise select New Tab Stop to continue adding tabs. Once you are done, make sure to click on Apply with the appropriate options selected.