In the construction "[get] + [direct object] + [past participle]," the past participle is always used.
I'd like to add that a present participle (V-ing) may be found after the direct object in very few, highly idiomatic cases like "get it going."
That's very true, Gustavo. Naturally, I thought about that shortly after making my generalization. I thought about adding an addendum about "get it going" ("get it running," "get someone going on something," etc.), and came very close to doing so, but bear_bear's English is so weak that I didn't want to complicate things for him, especially with the change from passive to active meaning. They might properly be considered two entirely different constructions.
As you say, it is absolutely impossible for the present participle to be used in bear_bear's example, and the "get it going/running" construction is not very productive, except with lexical variants in a couple of cases. A car mechanic can get a car running, a plumber can get the water flowing through a pipe again, and we can get each other going on exciting grammatical constructions. Much more usual is the case of getting something or someone to do something.
We can say, "We got bear_bear to understand the construction."
We can't say, *"We got bear_bear understanding the construction."