I had to do a clean install and backed up the entire hard drive beforehand,…but now I can’t find my old database - I am trying to import the previous db into DT Pro
Thanks ahead of time 
Was the database you are seeking created by DEVONthink Professional?
If so, it will be located in your User directory, ~/Library/Application Support/ and will be the folder named DEVONthink inside the Application Support folder.
Let’s copy that database to your hard drive and convert it to a DT Pro database:
[1] Select (don’t open) the folder named DEVONthink. Copy it to the Documents folder on your hard drive.
[2] Select (don’t open) that copy of the database. Press Command-I to open the Info panel. Rename it. For the sake of illustration, rename it “Harry.dtBase” (without quotes, but case in the suffix is important). Press Return and close the Info panel. You should now see a package file with the DT Pro shell logo. It’s now a DT Pro database file.
[3] Double-click on the new database file to open it under DT Pro. Is it a working database? If so, success. Now run Tools > Verify & Repair, followed by Tools > Backup & Optimize. It’s ready to be used, or you can export data from it and then import that data to a different database.
No, it was created in Devon Note…trial version
In that case, you should find the DEVONnote database in the same relative position as that described above for a DT Personal database. It will be the folder named DEVONnote.
However, I would suggest that to transfer material you created in the DN database you should open the database under DEVONnote, select the material you wish to transfer and export it to a folder (created for that purpose) in the Finder. Then in DT Pro choose File > Import > Files & Folders and select that target folder.