Raphael,
Thanks for posting the dgn file, that helps a lot.
Getting the x.y coordinates of the cells in the file is no problem. Set up an ElementEnumerator either from ScanCriteria, Selection Set, or Fence. Iterate through the ElementEnumerator to extract the cell element information. The Name and Origin properties of the cell element will give you the information you need.
Now comes the hard part. Figuring out the grid location. To do this you will need to define a grid origin. In the case of the sample dgn this is in the upper right at x=463600.001,y=326400.001. You will also need to define a grid interval (the space between tick marks). In the sample dgn this value is 12000mm.
To find the grid location you will need to subtract the cell origin x and y from your grid origin x and y to get your deltaX and deltaY. Divide deltaX and deltaY by the grid interval and round up to the next whole number. This will be the number of grid squares over and down. From there you can figure out your grid row and column.
To write out the information to a txt or csv file this tip will help with the syntax required.
envisioncad.com/.../readingwriting-text-files
Rod