Model Text cannot go along a curved wall, It can get hosted on straight or inclined faces of walls or any other elements.
To place the text along the face of a wall, select the Model text command, choose: set work plane ...click on the face of the wall...then the text will place correctly.
You can set this workplane on any other faces, including inclined faces but not curved faces.
Check this link for a workaround I have given
http://forums.autodesk.com/autodesk/attachments/autodesk/2103/85/1/MODEL-TEXT-ALONG-A-CURVED-WALL.pd...
Hope it will help
Aruna
School of Architecture
Ramaiah Institute of Technology
Bangalore