I found something that may be a bug.
When I install a wordpress site with another table prefix, for exsample, abc , and then I install the domain mapping plugin.
You will find that the plugin won't work. In mysql database , you can't find the table named like domain_mapping .
Is it a bug ?
I have imported another wp database and change the table name from wp_domain_map to abc_domain_map, the plugin works now.