Since I'm not sure what you've tried, here are several things to try.
1. baking soda - mix with water and microwave for a few minutes
2. lemons - slice, mix with water and microwave for a few minutes
3. vinegar - wipe on directly, let sit for a few minutes, wipe clean or mix with water and microwave for a few minutes
4. Clorox Cleaner w/bleach - spray on, let it set for a few minutes, wipe away
5. enzymatic cleaner (pet odor eliminator, urine remover) - spray on, let it set for a few minutes, wipe off
6. liquid potpourri/scented oils (any smell you like) - mix with water, microwave for a few minutes
7. fresh coffee grounds - mix with water, microwave for a few minutes
8. tea bags - mix with water, microwave for a few minutes
9. activated charcoal (can be purchased at Walmart or any pet store) - no need to mix with water, put in bowl, leave over night
10. Borax - mix with water into paste and cover walls, let set for a few minutes and wipe clean OR mix with water and microwave for a few minutes
11. nail polish remover (cleans yellowed interior) - wipe inside of microwave with soaked cotton balls, let sit and wipe clean
You may have to repeat a process or use a combination of them. Most of the items that are mixed with water need to be heated to a boil to work the best, but you can try just getting them hot and leaving them sit. Try the methods that involve items you already have at home first. Check the microwave every few seconds to make sure that what you are heating isn't getting too hot or burning. Microwaves vary in the temperature of the settings because some have higher voltage than others. High temp on one microwave may be several degrees hotter than another. So, some may only take 2-3 minutes and others may take 4-5 minutes. You don't have to stand in front of the microwave for the whole process, but keep an eye on what's cooking. Hopefully one of these methods will work. Good Luck!