"They put on dresses."
Translation:Ils mettent des robes.
In my opinion "Ils se metten des robes" means that "they put on the dresses " or they will be wearing the dresses. However this was not accepted.
Instead the valid translation of Duolinguo is "Ils mettend des robes" which in my opinion can be interpreted as if they put dresses in any place, in boxes, or else, this is not "put on" the dress.
Can any one explain please?
The verb mettre means "to pun on/slip into" when the direct object is clothing and there's no prepositional phrase to indicate they are putting something somewhere. Neither metten nor mettend are valid conjugations of this verb. The construction "Ils se" is only used with pronominal verbs (which mettre isn't) or in passive impersonal sentences (which this isn't).
If you got the English to translate into French, either "Ils" or "elles" would be grammatically correct. It isn't hard to imagine a situation in which males might be putting on dresses, if that's what was bothering you.
I believe the rest of your post would be better rendered as "...mais je ne suis pas certain. Est-ce que j'ai tort?" "Chui" is very slangy, mostly just spoken or used in texting. Probably not appropriate in this learning environment.
Here's an interesting article about texting in French: http://french.about.com/library/writing/bl-texting.htm