how do i download the pdf pak i bought
You don't. It isn't something you download, you just sign in and use it.
But it doesn't show even when signed in the option to combine/merge files.
When you sign in to https://cloud.acrobat.com/createpdf, you should see this. Simply click on Combine Files and follow the prompts.
For more instructions, see: Document Cloud Help | Using Adobe PDF Pack
If you're starting in Reader DC, click on Tools (upper left) and then choose the Combine Files tool:
Hope this helps!