Creating and modifying shares requires SMB protocol is turned on.  First ensure SMB is on via Storage Settings menu

Expand the pool options by clicking the arrow at the bottom of the pool

Click on "Shares".  Datasets, which are individual file systems will be shown.  Note: you may only have one dataset as often this is normal for a system. 

Click the small arrow to the left of the dataset or file system you are interested in creating or modifying

If you wish to create a new share you select from the larger OPTIONS button, if you want to modify an existing share you choose the OPTIONS button for each share.

Add Share, an add share wizard pops up:

Modify Existing Share, a modify share wizard pops up:

Changes take affect as soon as you exit from the last menu in each wizard.