Click on the arrow tool in the toolbar and then click on the note button in the object type palette. Control click the two notes between which the slur is to go. Choose Slur from the Edit menu that appears, then Above or Below from the submenu.
You can also create a slur as follows:
Click on the pencil tool in the toolbar and then click on the symbol button in the object type palette. Click on the symbol button again to open up the symbols palette. Click on Adjust to open the adjustable symbols palette. Click on the slur, the symbol at the bottom left of the palette.
Now click and hold down the left mouse button where you want the left edge of the slur to go. Keep the mouse button down and drag the mouse to where you want the right edge of the slur to go.
Release the mouse when you have positioned the slur approximately where you want it. There will be three little diamonds at the left, right and center of the slur. You use these control points to further shape and move the slur. Click on a control point to select it and drag it around to shape or move the slur. Alternatively you can move a control point around using the arrow keys. You can move between control points by using the TAB and the SHIFT+TAB keys as well as by clicking on them with the mouse. The middle control point is used for shaping the slur and the outer control points for setting the start and end points of the slur. When you are finished shaping and positioning your slur, click the right mouse button or press the ENTER key to enter it into the score. If you don't want to enter your slur, press the ESCAPE key.
When you are entering the slur symbol, you can automatically position the left slur point over the closest note by pressing Z. You can automatically position the right slur point over the closest note by pressing X. You can automatically center the slur by pressing V. If the slur is above the notes, the slur end points go over the closest notes when you press Z and X. If the slur is below the notes, the slur end points go under the closest notes when you press Z and X.