Each 5 bit segment is a number. Starting on the right hand side they start worth one then double as you go left. Like this:
VALUE--16-8--4--2--1
BIT------1--0--1--0--1
You add up all the numbers with a 1, so in this case 16+4+1 = 21. So 10101 = 21
If you want a letter you then have to map that number to an ASCII table like this one: http://www.asciitable.com/
That will tell you that 21 = Negative Acknowledge, which is used for transmitting data. Numbers run 48-57 and letters 65 to 90 and 97 to 122 and ASCII is an 8-bit code, so you should be using 8, not 5 bits really.
My name is Iain. According to the ASCII table that's 73 (I) 97 (a) 105 (i) 110 .
To turn numbers back to binary, just work out that bits you need to set to 1 to add up to your number.
Apparently so, however the chap who claims to have received the message makes his story a little better everytime he tells it. I lead a sad existence reading everything and watching everything I can about UFO's.