Autodesk Revit Architecture
There is no such tool. As Scott said, the dimensions in Revit are intended for continuous operation. Each consecutive click finds a new point for the next dimension segment, and each successive dimension is grouped with the first into a single entity until the tool is restarted.
The link that was posts did not have images, but it did have two good videos. The "Dimensioning Tools" video will give you a good idea of the intended workflow with Revit dimensions.
As others have said, dimension strings are automatically continued when you are first creating them. However, if you've finished a string (accidentally or intentionally), but would like to extend it further, select the string, then in the Modify tab choose "Add witness lines", then click on whatever else you want to dimension to.
Similarly, if you've finished a dimension string, and want to remove some items from it without recreating the entire string, select the string, then right-click on the little blue dot that is about halfway between the dimension text and the object being dimensioned, and choose "Delete witness line".
Better yet, read the wikihelp on dimensioning.
