From the online help:

To add Polar Snapping Mode to the Snaps toolbar:

  1. Open the Snaps toolbar, if necessary. To do so, right-click an empty part of the main toolbar, such as the area directly below one of the drop-down lists, and choose Snaps.

  2. Drag the right end of the toolbar to the right to make room for a new button.

  3. Choose Customize menu > Customize User Interface.

    This opens the Customize User Interface dialog.

  4. On the dialog, click the Toolbars tab.

  5. Scroll down the Actions list on the dialog to the Polar Snapping Mode item. You can jump to the P section by clicking any item in the list and then pressing P on the keyboard.

  6. Drag the Polar Snapping Mode item from the list to the empty section of the Snaps toolbar. This adds the button to the toolbar.

  7. Close the Customize User Interface dialog.

    3ds Max automatically saves the toolbar in its revised state and makes the new button a permanent part of the user interface.

To use Polar:

  1. Turn on the Polar Snapping Mode button.

    The button appears depressed.

  2. Begin to draw a line spline.

    An orange compass appears where you place the first point, along with a red number indicating the angle of the current line segment with the positive direction of the local X axis.

  3. Move the mouse cursor around in the viewport.

    The line jumps to specific angle increments from the X-axis, while the compass reading updates to show the current angle. You set the angle increment in the Grid And Snap Settings dialog > Options panel > General group, which you can access by right-clicking the Angle Snap Toggle button on the main toolbar.

  4. Click to place the next vertex.

  5. Repeat steps 3 and 4 until you're ready to complete the shape.

  6. Do any of the following:

    • To finish the shape at the most recent vertex without closing it, right-click anywhere.

    • To finish the shape by closing it, position the mouse cursor close to the first point and then click. A small dialog opens asking if you want to close the spline; click Yes.

    • To finish the shape by closing it while constraining the line to the polar snap, first position the mouse cursor close to the first point, press and hold Alt to constrain the mouse by both the previous point and the first point, and then click. This vertex is automatically placed at the current angle increment from the first point, so that you need only click the first point to close the shape.