Hi Uma,
In RSD1 , you need to goto button "Master Data/Texts" (next to Business Explorer button).
There you need to enable (check box) "Medium length text exists".
Please save the infoobject after enabling this check box.
Now if you goto button "Business Explorer" , you will see that "Medium Length text" option is available for selection for "Text Type".
Please make sure you activate the infoobject and not only save it.
Regards,
Amit