You might find some of the articles here to be helpful:

https://makemusic.zendesk.com/hc/en-us/search?utf8=✓&query=Dulcimer

A more informative topic heading would probably be helpful, as well. 

Welcome to the forum!

If I understand you correctly, you are talking about a 3-string instrument with diatonic fretboard (not chromatic fretboard) - right?

Are you an experienced user?

I am not sure how detailed instructions you need.

One way to do it:

File menu > New > Default Document

In the Default Document there are ready made tablature instruments, including Dulcimer - DAD.

The new, empty document has only one staff, in treble clef.

In this example let us say that you will add a staff below, with tablature for Dulcimer - DAD.

Window menu > ScoreManager > Instrument List

Next to Add Instrument, set the Score Order to Custom.

Add Instrument > All > Tablature > Dulcimer TAB - DAD Tuning

This step will add a Dulcimer Tablature staff below.

Unfortunately the Default Document has an error in the Dulcimer diatonic fretboard.

You will have to correct it:

In the Instrument List, next to Notation Style: Tablature, click on {Settings…}.

You get to the Tablature Staff Attributes. Click on {Edit Instruments…}.

You get to the Fretboard Instrument Definition where you can set the number of frets to 7 or 14.

Getting the 6+ or 13+ will require a workaround.

In the Fretboard Instrument Definition, next to Diatonic, click on {Define…}.

In the Default Document the Dulcimer’s {two octaves} diatonic fretboard (= 14) is defined as

2,2,1,2,2,2,1,2,2,1,2,2,2,1

Change this number sequence to the correct definition which should be

should be 2,2,1,2,2,1,2,2,2,1,2,2,1,2

Now for the workaround:

If you have a 2 (representing 2 half steps) and enter the chromatic pitches there, they will both have the number 6, and they will both play back at the correct pitch. All that is needed is then adding the + sign.

To add the + sign I would use the Expression Tool.

I hope that this is clear?

If not, ask again.