What version of Premiere Elements are you using and on what computer operating system is it running?
Track Matte, as mentioned by SG is a popular way to blur a person in a video. But there are other ways. One would be Effects Mask feature.
If Premiere Elements 11 or 12, basically
Apply Effects Mask (Clip Menu/Effects Mask) to clip on Timeline.
Drag Fast Blur or Gaussian Blur effect from fx Effects Blur & Sharpen category into the Effects Mask on screen
Do your editing of the effect via Applied Effects Tab/Applied Effects Palette/ and Fast Blur or Gaussian Blur expanded panel
Keyframing of the Effects Mask can be done.
Please let us know if you have questions or need clarification.