If you want all the notes in a track to go up or down, you can set Use Ascending or Descending Stems to the direction you want from the Display Track dialog, available under the Display menu.
If you want to enter two voices, one with stems up and one with stems down, proceed as explained in the section on displaying independent voices in a bar on the same staff.
If you want only a selected group of notes to have stems a certain direction, select these notes, and set the stem direction by choosing Group, then either Big Notes or Small Notes and then the stem direction you want. Doing this will also group together the notes that you have selected, so make sure that you select the notes in beam groupings that you want. If you want to set the beam groupings back to the default (grouped by beat) select Default Group.