A question of Nala
In Nala Ch. 5, sentence 4, reads the sentence:
सुष्णन्ती प्रभया राज्ञां चक्षूंषि च मनांसि च
according to the rules, the present part. of irregular verbs, when forming the female form, should not add a "n", but why here the word "sushnanti" with a "n" between "a" and "t"?

Question

A question of Nala
In Nala Ch. 5, sentence 4, reads the sentence:
सुष्णन्ती प्रभया राज्ञां चक्षूंषि च मनांसि च
according to the rules, the present part. of irregular verbs, when forming the female form, should not add a "n", but why here the word "sushnanti" with a "n" between "a" and "t"?

Answer

Two things. 
It is supposed to be "muSNantI" rather than "suSNantI". 
As for the dental nasal in the ending -antI, you are correct. Verbs of the second conjugation form their feminine present participles have only stems with no -antI but with only -atI. 
However what most likely the situation is that this is an aberration of which there are loads in Epic Sanskrit texts. 

Please see "A Grammar of Epic Sanskrit" by Thomas Oberlies. There he has compiled such exceptions. Please see page 70 for a list of such stems with -nt- for even reduplicated class 3 verbs (jAgrantam as opposed to jAgratam and so on). 

Vedic texts also show variations in general. 
What happens is that when learning Epic Sanskrit we think it is all very strict and uniform. But it is not so. 
Just take it in your stride and it will all be okay.