How do i hook up usb button in intel 845 motherboard

Well, that's a start. You know you have an Intel motherboard in your eMachines, that uses an Intel 845 motherboard chipset.

It is an Intel D845GVSR motherboard.

http://www.intel.com/support/motherboards/desktop/d845gvsr/sb/cs-008823.htm

Left-click on the blue -
Intel Desktop Board D845GVSR Product Guide (PDF)

This is a PDF file. The computer you are using now has Adobe Reader on it, which uses PDF files.
(I would advise upgrading to Adobe Reader X { 10 }, however, if you don't have it )

After you click on the above link, it may take up to 30 seconds before the first page comes up. (Took 6 seconds just now. Adobe Reader X, and medium speed DSL connection)

At the top of the PDF file, is the PDF file page number box.
It is to the left of the Down Arrow.
Go to Page 11.
T is the Front Panel USB header.

This is where the USB ports on the front of the computer, (Front Panel) connect to.

Go to Page 31.
E is the Front Panel USB header.
The Front Panel USB header is J9F2.

Observe the numbers next to the pins.
Go to Page 33.
This is the pinout for the Front Panel USB header, using those contact pin numbers.

1, 3, 5, 7, and 9 = one USB port
2, 4, 6, 8, and 10 = one USB port

Notice pins 9 and 10 are not used.

You can connect two USB ports to this header.

The pinout of ANY USB connector, female, or male, is set to a Standard.

This is the standard pinout for USB connectors,

http://pinouts.ru/Slots/USB_pinout.shtml

In case you cannot download the Intel D845GVSR motherboard manual;

Looking at the motherboard in it's normal position, installed into the computer case, the USB header is on the Bottom edge of the motherboard.

It is to the immediate Right, of the bottom white PCI slot (PCI 3)
It is to the Left of the motherboard mounting hole.
It is Below the Intel 82801DB, Southbridge chip.

Two rows of pins.
5 across the Top row, 4 across the Bottom row.

STARTING on the Left side, (Towards the white PCI slot), and going towards the Right, the pins on Top are numbered,
Pins 2, 4, 6, 8, and 10

STARTING on the Left side, (Towards the white PCI slot), and going towards the Right, the pins on Bottom are numbered,
Pins 1, 3, 5, and 7

Pins 1, 3, 5, and 7 = One USB connection
Pins 2, 4, 6, and 8 = One USB connection

Pin 9, and Pin 10 are Not used. (N/C. No Connection)

A) Pin 1 is for Power

B) Pin 3 is for D-
The Data, or Signal wire. It is the Negative connection

C) Pin 5 is for D+
The other Data, or Signal wire. It is the Positive connection

D) Pin 7 is for Ground

Going to the other USB header;
A) Pin 2 is for Power

B) Pin 4 is for D-

C) Pin 6 is for D+

D) Pin 8 is for Ground

For additional questions please post in a Comment.

Out of curiosity which eMachines is this?

Regards,
joecoolvette