What is the value of resistor R212 on a mackie thump 15-A powered speaker

SOURCE: hi, I need to know how

After removing the back cover disconnect the tweeter cable form the terminal. Then hold the tweeter magnet and turn it anti clock wise. The driver unit will unscrew and come of the horn assembly.
Keep the driver unit on a firm table and unscrew the four + screws. It has a sandwich assembly which will part into two if you put a sharp knife and twist. You will see the tweeter diaphragm which can be lifted up again by using a sharp knife. The replacement diaphragm is available from the below given address
http://reconekits.com/tapcothumpth-15ahorndiaphragmpn0013252-1.aspx

SOURCE: How do i connect thump

You failed to give us the most important information... what brand and model of the mixer? Youwill use balanced interconnect lines to a balanced MAIN output jack, assuming it exists...

SOURCE: i have a mackie fx 8 mixer connected to two thump

Balanced audio interconnects (either XLR or TRS cables from the mixer and then balanced audio interconnects from the amp daisy chain inputs to the existing speaker inputs. If the amps do not have linked input jacks, then use TRS type WYE connectors at the inputs. Then use speaker rated cables from the amps to the passive speakers.

SOURCE: is it normal for a thump when turning on active mackie speakers

Yes, it's normal. You can probably turn the speaker volume down to minimum before turning them on to minimize it.