There are two different types of head for this strimmer. I assume you mean the bump head (Reel Easy String head) rather than the pro cut II head.
To insert line in the bumb head you need a line 7.6 metres in length. Rotate the knob on the head until the slot on the knob lines up with the two arrows on top of the string head. Push the line in the eyelet on one side of the string head until it comes out the other side. Get the line even on both sides then turn the knob anticlockwise to wind in the line. Leave about 20 cm protruding at each side.
If you are using the Pro Cut II head then you need line 2.4 to 2.7 mm in diameter. Cut lengths 28cm long. Push the line into the slots on the side until it comes out of the hole on the top.
You can find the manual you need online but it took me ages to find it and I can't remember the url. If you can't find it contact me at jaydax.co.uk and I'll send you a copy of mine.
Jaydax, May 2009