Click the lowercase GreekLower_greek.tex buttonitbpF19.5625pt18.8125pt2ptlgk.wmfor the
uppercase GreekUpper_greek.tex buttonitbpF19.5625pt18.8125pt2ptugk.wmfon the Symbol toolbarSymbol_toolbar.tex.
Click the character you want.
Some Greek characters are available from the Common Symbols toolbarCommon_symbols.tex.