Just make sure you have done all these and check CAREFULLY:
- Make a new canvas with the Width and Height of '513x513' NOT '512x512' (That's for presets)
- Go onto advanced options making sure you have seletcted 'Gray scale' (RGB colour is for your mask)
- Once you have done your map save it as [Map Name].PNG (NOT .jpg or any other or it won't work)
After that go onto your map maker and type in the name of your map and make sure it's the EXACT name from what you saved it as.
Hope this helps, if not, tell me about the errors. Maybe share a few screen shots?