I would guess that what you have done is embedded an image rather than actual text. to get the text into a doc from a scan you need to use Optical Character Recognition (OCR).
If its an image then you can't edit it in word. depending on what you are trying to do if its just an odd word you may be able to edit it in an image editor and add text that way. e.g. if its a form and you just want to add your name etc.
If you want to do 'proper editing then OCR is the only way.
Most scanners come with soem freeware to do this, you may have not installed it.
If you haven't, Google manage teh following project which should do what you want.
tesseract-ocr - Google Code
I would say its often not 'perfect' and you may have to do some tweaking.