Float Values (I’m new to Openplan)

I am having troubles with float values in projects. Let’s say I place a Finish No Later Than date on the critical deliverable. If the activity is negative the float runs through the project as I would expect. However when there is positive float on the critical deliverable the float value in the project is zero, even when finishing early.
The bottom line is if anyone can offer basic steps to determine the longest path (or critical path)with relative float values I would appreciate it greatly.

thanks again,
Wayne