I would

1) Create a new document with the desired orchestra - with correct key and transposition in each instrument.

2) Copy/paste the music from the piano score document into the orchestra score document.

Remember that you - in the Document menu - can select “Show Active Layer Only”, to copy only one layer.

As we found out in another thread, you are using PrintMusic 2014. The templates Peter referred to above don't install with PM. They do install with the demo for 25, however, and PM 2014.5 can open them (the Orchestra template is 24 staves, the max that PM can work with). You'll have to navigate to them, not hard. The path will depend on whether you're running Mac or Windows.

The Finale 25 demo is good for 30 days and if you want to upgrade, MM has special pricing of $199 right now.

If you are running a 32 bit Win OS, I don't know if the F-25 installer will work. If not, the templates are only 3.5mB and can be emailed. 

I just bought "Print Music" three days ago, why wouldn't I have the latest 25.4.1?

No. PrintMusic is a different product. The current version is 2014.5. It is still being offered for Windows but has been discontinued for the Mac because of its incompatibility with OS 10.13 High Sierra and future versions of the Mac OS. Here's the feature comparison sheet:

https://www.finalemusic.com/products/compare-finale-products/

One of the things you'll see is that PM only installs 7 templates while Finale 25 installs 68. As a number of us have found, PM can open and use the Finale templates.

I have a working copy on my iMac running OS 10.13 but I upgraded the OS. My understanding is that it will not install properly on a Mac running High Sierra.

If you meant to buy Finale 25, you can use the Submit a request link at the top of the page to discuss this with MM Sales—there is a special upgrade price that makes the overall purchase cheaper if you bought PM first.

Those of us with Community Expert as part of our signatures are users like you. We're not affiliated with MakeMusic; just experienced folks who post a lot.