Arpeggios are set by dragging an arpeggio symbol from the Arpeggio & Glissando palette to a note of a chord.
To change the length of the arpeggio, double-click the arpeggio and drag the handle up, or down.
Glissandi are set by dragging a glissando symbol from the Arpeggio & Glissando palette to the first of two consecutive notes on the same staff.
Edit, or delete the text of a glissando by right-clicking it and select "Glissando Properties" in menu or in the Inspector. If there isn't enough room between two notes, MuseScore will not display the text.