What is the point in keeping China out of the Missile Technology Control Regime (MTCR) if the regime's aim is to control the technology?
According to Wikipedia, China was rejected in 2004:
In 2004 China applied to join the MTCR, but members did not offer China membership because of concerns about China's export control standards.
Checking the cited sources shows the Arms Control Association saying:
In the weeks preceding the MTCR meeting, the United States imposed proliferation sanctions on eight Chinese companies. One of those, Xinshidai, was specifically accused of missile proliferation. The others, two of which the Bush administration previously penalized for missile proliferation, were punished for unspecified deals with Iran, which Washington charges is covertly seeking nuclear weapons and developing ballistic missiles to deliver them.
While China denied these accusations, their denial suggests that it is the consensus of the MTCR countries that the allegations are true.
It's also worth noting that the MTCR prohibits members from selling and purchasing technology from non-members. So while China is not in the MTCR, they can't sell their missile/rocket technology nor buy from members. If China were already in the MTCR, then the other countries would have less ability to insist on better behavior from China. And of course, China would become a potential proliferator of technology that they bought from other members.
For this reason, China needed (and may still need) to first demonstrate that it will comply with the non-proliferation requirements. Only after it has proven itself would it be allowed to purchase missile technology and finance its research and development by selling missile technology to other members.
I would also suggest that if China really wanted to join the MTCR, it would be looking for more ways to put pressure on North Korea to give up its ballistic missile and nuclear weapon programs. As North Korea's primary trading partner, China is widely seen as the country with the most influence. If China wants the benefits of joining the MTCR, it will also need to start bearing some of the costs.