Press the ctrl-key and click the or button in the Proposals area to call the "Define Previous/Next button" dialog.
If you prefer to use the menu, go to "Define Navigation Button" in the Tools menu.

As the title of the dialog in the picture shows you, I have clicked on the "Previous Sentence" button.

The dialog lists all sentence types with their color. By default the option buttons are all set to "Take". This means that Tstream Editor Studio will actually take you to the previous or next sentence whether you use the buttons or the shortcuts.
Note that in this case, the "Revisions" option buttons are not available because I'm defining the settings for a document that has to be translated.

When you start translating a document, you don't have to concentrate on "Context Matches", "100% TM Matches" and "Repeated Matches". They are supposed to be correct. So, you can safely skip them by clicking their respective "Skip" option buttons.

Skipping sentences only affects the way the "Previous Sentence" and "Next Sentence" action work. You still can go to any sentence in the document e.g. by Ctrl-clicking the sentence in one of the preview panes.

By unchecking the "Apply also to Next/Previous button" you can create a different behavior for the opposite navigation direction.

The definition of the navigation buttons is document related and is not saved.

And just in case... don't forget to revert all option buttons to "Take" when you do your final proofreading.

Next time: some more tips on navigation.