Aorist Tense:

Simple fact.
Once and for all.
Specific.
 

2nd Aorist Tense Forms: Participle

ὁ δὲ ἀδελφὸς πρὸς τὴν θύραν ἐλθὼν 
      καὶ τὸν Διακιόπολιν ἰδών, λέγει....
Coming to the door and seeing Dikaiopolis there, he says...
The meaning here is: after coming to the door and after seeing Dikaiopolis, he says.
The Greeks loved using participles, we don't, because in English participle constructions sound rather clumsy. So we would rather say:
The brother comes to the door, sees Dikaiopolis and says.....
You have to remember that the aorist always means: simple fact, specific action and does not necessarily refer to the past, except in the indicative (stating past facts). The action expressed by the aorist participle does however usually take place first, the other action, whether in the past, the present or the future is the last in any sequence:
μαθών τὴν ἀγγελίαν, εἶπεν .... When he learnt about Philip's adventure he said....
μαθοῦσα τὴν ἀγγελίαν, εἶπεν .... When she learnt the news she said....
μαθοῦσα τὴν ἀγγελίαν, λέγει .... When she learns the news she says....
μαθοῦσα τὴν ἀγγελίαν, λέξεται .... When she will learn the news she will say....

Further examples from the text:

οὕτως εἰπὼν εἰσάγει αὐτοὺς εἰς τὴν οἰκίαν 
So saying he leads them into the house. The saying comes first, the leading comes after.

πάντα τὰ γενόμενα αὐτῷ εἶπον 
They told him all that [had] happened = that had happened before they told it.

 
active participle        middle participle
      NO past tense marker                NO past tense marker 
 μαθ    2nd aorist stem   γεν    2nd aorist stem 
   ών 
   οῦσα 
   όν  		  active participle marker (m)
  active participle marker (f)
  active participle marker (n)
    όμενος 
   ομένη 
   όμενον 		   middle participle marker (m)
  middle participle marker (f)
  middle participle marker (n)
 
Why is there an acute, not a circumflex on the aorist participle ending -ών?
 
Because only long diphthongs and contractions take a circumflex on a long last syllable:
seeing: ὁρῶν [ὁρά-ων] versus ἰδ-ών
the contest: ὁ ἀγών, τοῦ ἀγῶνος
the voice: ἡ φωνή, τῇ φωνῇ, τῶν φωνῶν [φωνά-ων], ταῖς φωναῖς.
 

2nd Aorist Participle Forms

Present   2nd Aorist   Meaning
Participle Indicative Participle Participle
 ἄγ   ων   ἤγαγ   ον   ἄγαγ   ών  leading, taking, ...  
 αἱρ   ῶν [έων]   εἷλ   ον   ἑλ   ών  taking, capturing  
 βάλλ   ων   ἔβαλ   ον    βαλ   ών    throwing, pelting, ...  
 εὑρίσκ   ων   ηὗρ   ον    εὗρ   ών  finding, having found  
 ἔχ   ων   ἔσχ   ον    σχ   ών  having, holding  
 λαμβάν   ων   ἔλαβ   ον    λαβ   ών    taking/having taken  
 λέγ   ων   εἶπ   ον   εἰπ   ών  saying, telling.  
 λείπ   ων   ἔλιπ   ον    λιπ   ών  leaving, ...  
ὁρ   ῶν [άων]   εἶδ   ον   ἰδ   ών  seeing  
 παρέχ   ων   παρέσχ   ον    παρασχ   ών  giving, handing over  
 πάσχ   ων   ἔπαθ   ον    παθ   ών  suffering,experiencing  
 πίν   ων   ἔπι   ον    πι   ών  falling, ...  
 φέρ   ων   ἤνεγκ   ον    ἐνεγκ  ών  carrying, bringing  
 φεύγ   ων