I was having similar trouble, but I'm pretty sure I just figured it out!
To change the pull-down menu for the 'Activity' column in your 'Workouts' table,
1. Highlight all of the body cells in the column by selecting the first in the column, SHIFT, then select the last in the column.
2. Go to 'Cells' in your Inspector (fourth from the left at the top)
3. Under 'Cell Format', it should say 'Pop-up Menu'; if it doesn't for some reason, select that option.
4. Select the activities listed that you do not want to show up in your menu and click the minus sign to delete them.
5. Click the addition sign to add as many activity types as you'd like
This should apply to all of the cells you highlighted. You will also need to change the activities in your 'Workout Summary' table to reflect these changes in the table and charts. Hopefully this answers your question better.
Good luck!
Lauren