If you set Bitcoin Core to use D: as it's data dir when you did the install... then you need to find where you wallet.dat file is on D:
That is where you need to create "bitcoin.conf" file (it is just a text file)
Also, you don't "need" a config file, they're just useful for setting commandline options if you need to use them, but it isn't required for the application to run. If it isn't running properly, then you'll need to post the Bitcoin Core debug.log file so we can see what the exact error is
NOTE: Do NOT just post the contents of debug.log here... it probably won't fit in the max post size... instead use
www.pastebin.com and just post the pastebin link here
