Are you clicking on the toolbar icon? If not, How to launch the tools
Have the licence details been entered since installing? If not, How to enter the licence keys.
Are you getting asked to choose a database in the Open Database UI? If not, How to launch the tools.
Does the UI come up? If not, do you have control and can open databases still? If not, this may be due to a complex design for Delta to investigate and compare, and/or network performance. It may take many minutes to open the comparison on complex, large, remote databases. Try Delta on a pair of small local databases to make sure the UI appears. When comparing data, choose a view with as few documents as possible, preferably creating a dedicated, simple, cut-down view for the comparison and modifying it to perform further comparisons.
If Notes appears to be hanging then shut down Notes, run ndelta.exe from the Notes program directory. Select the same database in the Open Database UI as was open in the Notes client when clicking on the Delta icon, and see if the Delta UI appears? If it does then the icon may be faulty, see Reinstalling. If it doesn't then repeat using ndelta.exe and choose a small local database this time to confirm Delta is installed correctly.
Is it crashing or giving an error? If so, contact support.