Few possibilities
1-
$("[id$='myID']")
is not returning anything
2- if point # 1 is returning anything it doesn't contain MACAddress property
3- Point #1 is returning a collection of objects and MACAddress method belongs to an item not collection so fetch an object, if it should return only 1 object then get 0th index item and try accessing the item again.
break your statement in 2 parts
var extractedIDs = $("[id$='myID']");
if(extractedIDs != undefined)
var macAddress = exgtractedIDs.MACAddress();
Add some breakpoint and debug it.
By the way what do you mean by
"The dll file returns the MAC address of the client"
?
Thanks,
Hemant