That's an excellent question! There are two ways:
Select main menu "View" and select the tab "Views".
Select the "Screen" view.
Your complete score is displayed on a single horizon.
Using the input palette, you can tie or slur the desired notes now.
Select one of the elements from the input palette and subsequently draw a square around the desired notes.
If you prefer remaining in "Page view", you can simply select the desired notes:
Press "crtl" and select the desired notes. They will turn red.
A new window will open.
Pick one of the commands at display.
Once you've chosen a command, the tie or slur will appear in your score.