ive here my weapon i xsi, but its detail i son the right(wrong) side. so i want it to be mirrored, so its detail is on the left side. anyone knows how to do this?

Click the button next to duplicate that has two pieces of paper opposite to eachother

where?

Top right I believe, I can get a screenie soon and circle it
Your using Mod Tool 6 right?

yep(xsi 5 on other pc)

you can mirror an object with the modify>polymesh>symetrize polys command
you can also duplicate/clone the object freeze the transforems of the copy and scale it with -1 on the axis you want (usually x-axis)