You have to append each element stored (a line of text) from String[] into 1 regular String.
Since String appends are slow operations by nature, in my example here,
I'm gonna use class StringBuilder to intermediate the whole process.
After that, it's finally dumped into our final String variable.

Any doubts about how the code is implemented, don't fret to ask! 

Text used -->  "a Long Password.txt":

During a recent password audit by Google, it was found that a blonde was using the following password:

“MickeyMinniePlutoHueyLouieDeweyDonaldGoofySacramento”

When asked why she had such a long password, she rolled her eyes and said:

“Hello! It has to be at least 8 characters long and include at least one capital.”

/**
 * Glue loadString (v1.11)
 * by GoToLoop (2013/Feb)
 * for Telmo.Bacile
 *
 * http://forum.processing.org/topic/string-vs-loadstrings
 */

//import java.lang.*;

String[] txtLines = loadStrings("a Long Password.txt");
int numLines = txtLines.length;
println(txtLines);

StringBuilder gluedLines = new StringBuilder();

for ( int i=0; i!=numLines; 
gluedLines.append( txtLines[i++] ).append('\n') );

String wholeLines = gluedLines.toString();
println("\n" + wholeLines);

txtLines = null;
gluedLines = null;

exit();

Or just use join() which will do the job for you...
(But GoToLoop's answer is educative, so it is good too! )

I guess I should remember to always look at Processing's reference 1st. 
The StringBuilder class was the trick I've already knew. And I believe it's the fastest!
I wonder which implementation Processing's join() uses internally?!

Also, an enhanced loop alternative:

for (String txt: txtLines)    gluedLines.append(txt).append('\n');