Nearly all modern carburetors have tamper-proof mixture adjustments. Many have plastic covers on the heads of the mixture screws. Some others have screws with a "security" recess, so you need a special tool to adjust them. We need to see pictures to be able to tell which you have. More importantly, what makes you think it is a mixture adjustment problem, rather than a malfunction? Has someone previously been tampering with the mixture screws? Most owners make things worse, not better, by changing their adjustment.
You can see an image of a standard Husqvarna carburetor adjusting tool here:
http://ecx.images-amazon.com/images/I/71Hy-55HlPL._SL1500_.jpg
