Welcome to the world of :Z media.
Part A: The recording speed selector is located in 'Common Settings' - click on the white check mark in the green disc at the top of the opening screen (when you first start DVDFab). then look down the left panel to DVD - Write, and click on that.
a box there will have a number with an 'X' beside the number, and an arrow pointing down. Click on the arrow, and select a number this at least 1/2 the max writing speed of your blanks (preferably the new ones you will be receiving), and you have just set the recording speed (if you use the provided VSO burner).
Part B: Before you waste too much more time fooling with the Memorex, wait for the new disks to arrive, and when they do, try one in both machines after recording a film of your choice.
There should be a noticeable difference, now (ie. both machines should play the disk, unless one has a problem with the format of blank you bought - a quick check of its manual should answer that question).
The help screens provided with Imgburn should answer most of your questions. Give them a peek while you're awaiting your new disks.