f.ex. = for example
You can measure current by inserting the instrument on the - or the + connection, you'll get a reading either way, but it may show for example -6A, then just swap the wires around on the instrument to get the positive reading, 6A.
the small m's = milli -volts or -amps, like 0,001 volt or 0,001 ampere. You need to find the bigger numbers, like 20 V for the voltage, and 10 A for the current. Otherwise you may fry the intrument fuses (or the instrument itself). In the owners manual for the instrument there may be some instructions as well.
Regarding charging (or decharging) current, the ammeter on the bike does the same as when you disconnect a battery pole and insert your instrument, they will then both be inserted in the battery's wiring path. The reading on your instrument should roughly match the reading on the bikes ammeter.
Study the wiring diagram for your bike, you'll see f.ex. the bike's ammeter is inserted between the control box and battery, thus either measuring drain or charging.