Is the field on your form a column in your list? If it isn't then you won't see it as an option on either of those actions? Also - to clarify are you updating the item the workflow is running on? 

Hi Cassy.  Thanks for the reply.  Yes, the field on the form is a column.  Again, I'm just starting to get my feet wet with SharePoint and Nintex Workflow.  So I'm not sure what you mean when you ask if I'm updating the item the workflow is running on.  If you mean is the workflow updating the form that it is running against, then yes.  I'm not reaching out and changing another form.  I'm working on an approval workflow, using State Machine and Request Approval actions.  I have checkboxes on the form labeled Approved and Rejected.  I want to use these checkboxes to show the status of the approval process in the form itself and to also use it so the workflow knows where the approval process is at since I have up to two levels of approval.  Hope that adds some clarity and thanks again!

Hi yes I think I am closer to understanding.

Can you please add some screenshots of your form, your workflow and where in the process you want to make this change?  I might be able to quickly mock something up for you.

I'm starting to learn more as well.  I found out Friday morning that the reason I couldn't see my columns/fields in the workflow was because they weren't published in a way that they were to be made available to the workflow.  Unfortunately, I don't have control of publishing the form or the workflow and am reliant on another group to do that for me.  Being new to workflows, I didn't know I had to ask for columns to be published as well.  It's a learning process.  Now that I can see the fields in the action, I think I can get somewhere now.  I appreciate your willingness to help.  I will be posting more I'm sure and reaching out.

ah I think I know why - is this a forms library?  if so, then they will need to "promote" any fields you wish to use in the form as a column!

Yep.  It's a forms library.  I will remember this for the next form in my queue for development.

i literally hit this issue only recently - I forgot forms libraries even existed and I kept wondering why stuff wasn't showing up in my form!