My wife is not able to sign our tax form, what I do about this

Please give more information. Why is she not able to sign the tax form?

See page 6 of Pub 501:

http://www.irs.gov/pub/irs-pdf/p501.pdf

Depending on the particular situation you may be able to sign on behalf of your spouse and attach an explanantion (for example if she is deceased, or incapable of signing due to illness, or is in a combat zone), but in other cases you will need a Power of Attorney.