how do i continue to use my iMac when mouse is charging

You plug in the USB mouse that you should always have on-hand. :-) If you use a wireless mouse and keyboard it is wise to keep on hand a usb keyboard and mouse for situations like yours or if you need to do some troubleshooting such as your wireless mouse decides to not work one day. They are inexpensive and type will work just fine.

While there are other obvious reasons for having USB backup

keyboard and mouse, you can also get a charging cradle or

wireless charging pad to put the mouse at the end of the day to

keep it charged. Unlike the old NiCad technology, the new

battery tech can be kept at or near full charge nearly for ever.

As a matter of fact, deep discharge and recharge actually

lowers their life time.

You can also use Mouse Keys.

Control the pointer using Mouse Keys on Mac - Apple Support

Your welcome. You can find them lots of places, office supply stores,e-bay, retailers such as Macsales.com etc....

You're welcome.