If I understand you correctly, your steps are like this:

You are entering ready made expressions from the expression categories Tempo Marks and Tempo Alterations - right?

In the window Expression Selection (and in the Expression Designer) the expressions display correctly, with note symbols - right?

But as soon as you click the button Assign, the expression appears in the score as letters instead of note symbols - right?

Yea - If I go to "Tempo Mark" or" Tempo Alterations" then go to 

"Create tempo mark"(or alteration") then go to  "insert note" and click on, say, the quarter note - the letter "q" appears

any way to change this to regular notes?

Take a look in

Document menu > Category Designer > Tempo Marks

Click the tab for Music Font.

What font is set as Music Font?

If the font is set to e. g. Times New Roman (which is a text font, not a music font), then you will get the behaviour you describe.

It is set as Maestro Times - how do i set foe actual notes

Change the Music Font to e. g. Engraver Text T.

Or you could try the Maestro font (which is not the same as Maestro Times).

Peter - thanks - everything working now - isn't Finale set up to show notes rather than letters.....?

… isn't Finale set up to show notes rather than letters? …

Actually, this particular setting is a document specific setting, not a program setting in Finale.

When creating a new document Finale makes it as a duplicate copy of a template, so that the new document will have the same settings as the template, it was created from.

From what template did you create your document?

The templates I use are provided by a number of publishers  - orchestra, Band , Jazz band, etc.

also - is there a way to hide the tempo marks - if I want to create a gradual Rits. or accels- rather not show 

a number of tempo marks within one or two measures

… is there a way to hide the tempo marks …

Switch to the Expression Tool, and context-click the expression handle.

In the context menu there is an option to Show/Hide.

… if I want to create a gradual rits. or accels- rather not show a number of tempo marks within one or two measures …

Instead of adding a lot of Tempo Marks, why not use a single expression with an accelerando?

You can let Human Playback interpret the accelerando, or you can give the expression a playback effect, via an executable shape.

OK - I write for large performing groups - lots of channels -  so Human Playback sometimes gets "sticky" in the playback -

all good info - I'm saving all your comments - if i need any more advice - thanks 

went to the Expression tool  (on both my computers  ) Finale 25 & Finale 12 - can't find the show/hide function

also - the pages on my templates on 25 are very white & bright  - with small note heads- my Finale 12  has darker pagers with

larger notes - really great contrast - easy on the eyes-  any way to darken the pages on 25 ....?

Preferences > Palettes and Backgrounds will let you change the color of the "paper."

Show/Hide is an option in the context menu of the Expression. Right-click in the handle.