The time signature 6/8 can be defined in two ways:

A) Number of Beats: 2 - Beat Duration: Dotted Quarter

This will beam the Eighths as 3 + 3.

B) Number of Beats: 6 - Beat Duration: Eighth

This will beam the Eighths as individual Eighths.

By The Way:

Have you tried the context menu?

When you are in the Time Signature Tool, the context menu will contain all the common time signatures.

Choosing 6/8 in the context menu will give you type A) where the Eighths are beamed as 3 + 3.

Thank you!

Ok. I have to jump in here, similar question but with 12/8 time signature. Each measure has 12 eighth notes but they are all separate. I'd like them beamed together but can't seem to figure out how even with the example above. Could not find the context menu. 

Windows 8.1

Version: 26.2.2.494

Time Signature Tool.

Double-click the measure/staff.

Set the Number of Beats to 4, and set the Beat Duration to a dotted Quarter.

… Could not find the context menu …

While you are in the Time Signature Tool, try a right-click on the measure/staff.

You should get a context menu with all the common time signatures, including 12/8.

Oh my, that was simple! Thank you!