Argh, I'm having the same issue I've seen intermittently. Scenario:
- Install domain mapping as per instructions.
- Log in as sub-site admin.
- Go to "Domain Mapping", enter domain and click "Add".
- Nothing happens. Field remains empty no matter how many times I try.
Interestingly, I can usually resolve this by going to the main site and adding a mapped domain. It "sticks" there. Then I delete it, and I can add domains on sub-sites. But I'm sure that's not the best workaround...kind of a kludge.