hey there,
I have not burned a Dual Layer Wii game before, but I have tons of experience burning Xbox 360 games which require DL. It's basically the same formula:
1. Extract RARs (unless you downloaded a pre-extracted ISO)
2. Start IMGBurn, and choose "write image file to disc"
3. Before burning, make sure you choose 2x-2.4x write speed (cause the bigger the ISO the more delicate it becomes)
3a. Also you need to set the "Layer break". To do this, go to tools, settings, click on the "write" tab, and where you see "layer Break" on the right, there will be two options: "calculate Optimal" and "User Specified". Choose User specified, and type 1913760. Just to make sure, that is the correct layer break, your ISO should come with a DVD file. Use Notebook to open this file and it should give you the layer break number. save your settings and commence step 4.
4. Just to make sure, check the "verify" box to have IMGBurn check for errors after burn.
5. Now burn away!!
Note: Burning an ISO that is 7GB or above at any speed other than 2x-2.4x is only gonna give you a headache (lag, might not boot).
If you get lag even after burning it at 2x, take in the fact that you burned a coaster ( I know it hurts. Dual Layers are expensive) and burn it even slower.
Good luck!!!