You have to maintain your CTR of 0.5% on Google search to be in "normal" status.
Your keyword is "in trial" status because the keyword is doing badly in the Google search. To improve the Google search, you may try some new ads for the keyword. Write a killing ad!
The easiest solution is to increase the max. CPC.
From adwords help:
There are a limited number of in trial keywords per account, so when this limit is reached, additional keywords will be moved to on hold status with priority given to those keywords with higher predicted CTRs. Accounts that have proven to perform very well will be rewarded with a greater number of in trial keywords. The limitation placed on the number of keywords that may be in trial helps ensure that users continue to see the most targeted, relevant ads via the AdWords program.
The kernel of the answer is contained in the info you quoted, where it says:
Accounts that have proven to perform very well will be rewarded with a greater number of in trial keywords.
AWA