I inserted a blank disc to format it, but the drive does not recognize it. Or I formatted the disc, but InstantBurn 3 won't mount it.

If the disc is still not recognized after you launch InstantBurn 3, you may have third-party drivers installed in your PC. Insert the disc in the drive, right click on the drive icon in My Computer and select "Properties" from the contextual menu. If "FS_UDF" appears as the file system, then the disc is mounted using InstantBurn. If not, the third-party software is conflicting with InstantBurn.
If you have used third-party packet-writing software on your computer, then some files, such as UDF reader files, from this software may still be in your system. If these files have control of the CD/DVD drive before InstantBurn, then they are prohibiting you from using InstantBurn.