(WARNING! I am doing this from memory)
If creating a mulitline attribute from scratch, use the DDATTDEF command, and there is a check-box to make the attribute have multiple lines.
If you have existing attributes, they can be modified to become Multiline attributes no problem. While in the Block Editor, click once on the attribute, and in the Properties Manager - under the Misc category I believe - you can set the multiline attribute option to "YES". Once you save and exit from the Block Editor, remember to use the ATTSYNC command on the block you just modified so the changes take affect to the blocks already inserted throughout the drawing.
