As I stated earlier, I'm not familiar with this area, but here's what I put together.
There are two types of summons, one for civil procedures and one for provincial offences (there's also criminal but we'll ignore it for now). Don't confuse the different rules. In civil procedures (I'm suing you) you can find info here, see section 3(e). I point this out because you should know what not to do. When you go to the court house, the clerk may try and make you do this. This is wrong and you need to know the difference.
As I posted above, for your trial, under s.39 the justice can issue a summons. I'm not sure why that was not indicated to the Crown at your first trial. If you don't know the name of the officer, send a letter to the Crown asking for the contact info (name, badge number, place of work, etc.) of their witness as identified at trial. You can also ask them whether they are compelling the witness to attend the next trial. If so, you're done.
If not, then you would have to see a justice "on call" at the court house and get them to agree to signing the Form 109 which you will have completed in triplicate (one copy for you, one to be served on the cop, and one for the Crown). If the justice signs it, then you serve the cop at his work. I believe it has to be done in person, not by mail and not by leaving it for him.
I don't know the time lines but it should be done at least several weeks in advance of the trial.
As you go through the process, you may be told clarifications/process modifications by the justice. Be prepared for that. Also be skeptical of what the court clerk tells you, they are often wrong.