Aart:Congratulations on learning music as an adult and learning to write it using Finale!
About your question: (these work on a PC, and should work on a Mac as well) if you want six eighth notes beamed together in every measure where they occur, use the Time Signature tool, and DO NOT USE the standard 3/4 time settings. Instead, set the Number of Beats to "1", and the Beat Duration to "dotted half note," using the sliders. Then click the Options button, and in the lower window, check "Use a different time signature for display", and set the sliders to 3/4 time. Then all six eighth notes will be beamed together.
If, however, you want to do this only occasionally, ignore the preceding paragraph. Instead, use Speedy Entry, and place the vertical crossbar on the first eighth note on any beat, and press the "/" key, which will include the (2-note) group in the preceding beam.
Another approach would be to select the Mass Mover Tool, select the measure you want to rebeam, then pull down the Mass Mover menu, and click Rebeam > Rebeam to Time Signature > and set the time signature to 1 over a dotted half note, and click OK.
Any of these should work, depending on whether you're entering new music or modifying music you have already written.
Good luck!
-- Dan
