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This is on the top of the camera, towards the left if the back of the camera is facing towards you.  The flat end of the canister will be pointing upwards. You may find you have to rotate it a little in one direction for the fork attached to it to fully enter the film canister. This is normal. Ensure that the film properly engages the sprockets as marked in the picture.  The shutter might not fire the first time if it's not armed already, of course, in which case just wind the camera on. Don't fire the shutter once the frame counter reads 1; this is the first frame on your roll. The film speed dial is in the same place as the shutter speed dial; it's the silver dial around the outside, which can move independently.To change the film speed, pull the silver ring around the shutter speed dial upwards. While holding it there, rotate the dial until you have set your desired film speed.Notice that the MTL3 has both DIN and ASA settings; modern films typically give their ratings in ASA (which is called ISO on digital cameras). (For example, Fuji Velvia 50 is ASA 50, not 50 ° DIN, the latter being equivalent to an ASA speed in the high five figures.

Summary:
Lift the rewind knob. Pull it a little further and the back of the camera will spring open. Drop a 35mm film canister into the chamber on the left hand side. Push the rewind knob all the way down. Pull the film leader away from the canister till the tip is at the green index mark on the right hand side, next to the take-up spool. Close the back of the camera. Press the shutter button, then wind on your film. Repeat the above step until the frame counter reads 1, as pictured. Set the film speed on the film speed dial.