Greg Jenner was named acting assistant secretary for tax policy in the Treasury Department. He will be filling in for Pamela F. Olson, who tendered her resignation this week. Ms. Olsen, whose resignation will take effect after completion of the fiscal 2005 federal budget, worked on tax issues relating to pension funds. She said she wanted to return to the private sector to devote more time to her family. Mr. Jenner will be acting assistant secretary until President Bush nominates a replacement.