In the Options tab enable Recurse Subdirectories, which is enabled by default. Better yet, click on Reset Settings and confirm, as you may have changed other important settings.
//edit too slow again
That guide is for creating an Audio CD, i.e. a disc that plays in pretty much any device that can read a CD. If your player supports MP3 disc then you should use the Build mode to burn those MP3s to a disc.
Then your options are:
clean the drive with a lens cleaning disc and try again;
use high quality media such as Verbatim and Taiyo Yuden blanks;
get a new burner.
ImgBurn has command line switches that you can use to automate its operation (they're described in the Read Me file), but notice that ImgBurn is a GUI application and it won't run "hidden" from the user.
Make a disk image from the bootable floppy then fill the information in Advanced -> Bootable Disc tab in ImgBurn's Build mode. If you think that's too difficult then give me a link to the files (you can send in private message if you want) and I'll make it for you.
Some antivirus/anti-malware programs don't like the UPX compression used in ImgBurn's executable, so disable your antivirus/anti-malware (or add ImgBurn.exe to its allowed/whitelist) and check if the problem goes away.
Alternatively you could decompress ImgBurn.exe and it should work fine.