For other newbies as confused as I was:
To install Hubitat Package Manager, you copy this url [6/27/24: Updated URL from @zenbob post below]:
[https://raw.githubusercontent.com/HubitatCommunity/hubitatpackagemanager/main/apps/Package_Manager.groovy ]
Then in Hubitat,
1 you connect to your hub
2 on the left in the black bar, you click "Apps Code"
3 then you click the [edited 8/2026 for new interface] "Add App" box in the upper left
4 you get a new page, and you click the [edited 8/2026] 3 dots in the upper right, and select import
5 This brings up a pop up "Import Code from Website" and you paste the above url in the space where it says "Insert url to import text into code editor"
6 then you click import
7 then you click save
8 then you click the hamburger bar in the upper left and it will bring up the black bar on the left
9 click "Apps code" in that black bar, and it will bring you back to the screen you got in step 3, but the Hubitat Package Manager will be listed as an "Apps Code."
10 On the black bar on the left, click "Apps" (it is third from the top)
11 Click "Add User App" in the upper right. A box will appear that includes the Hubitat Package Manager. Click on that entry
12 Click done
13 You now have an Apps screen that lists Hubitat Package Manager as an installed app. You can click on it to use it.