I use Android on my iPhone 4, but I cannot download the apps from the market. It always said unsuccessful. What setting should I change?
As far as I know, the Android Market doesn't quite work on "iDroid" phones yet. It's due to copyright issues, Google hasn't given permission to the developers working on the project for the Google-branded apps, so the Google servers won't allow you to log in properly. That's explained in a little more detail in the iDroid User Manual.
Unofficially, there's some mention of being able to extract the market.apk file from some HTC Android builds and use it that way, over in the iDroid to-do list (see item #15). They also mention that third-party Market apps can be used and work without issue, if you download an .apk for one and install it.