Here's some code I wrote for the same purpose :-
You need to pass
1) an ALLEGRO_DISPLAY * - one you've created with al_create_display of course.
2) an ALLEGRO_EVENT_QUEUE * - one you've attached the keyboard and display to .
3) a char * that points to some memory - this where the filename gets stored.
so have something like :
char filename[MAX_LENGTH_FOR_FILENAME_STRING] = ""; .. in the caller
4) as you can see - there's two fonts but you could adapt it to be simpler and use just one font - in my example the fonts are true type fonts.
The WHITE and DARK_RED etc... are
#define WHITE al_map_rgb(255, 255, 255)
Loose the #include <feature_representations.h> line but make sure you replace all the symbolic constants with your own - ie MAX_LENGTH_FOR_FRD_STRING, QUIT_DUE_TO_PROBLEM, etc....
Perhaps you make use of this 
I know it's a bit an effort just to get a filename but it works here...
If you want to use it, let me know if you have any problems..
#SelectExpand
1#include <allegro5/allegro_ttf.h>
2#include <allegro5/allegro_primitives.h>
3#include <feature_representations.h>
4
5#include "helper.h"
6#include "globals.h"
7
8
9
10
11#define AAA_B 1
12
13
14int get_filename_screen
(ALLEGRO_DISPLAY *display,
ALLEGRO_EVENT_QUEUE *q,
char *filename,
ALLEGRO_FONT *big_font,
ALLEGRO_FONT *small_font
)
15{
16 ALLEGRO_EVENT event
;
17
18 int current_character
= 0;
19
20 int string
[MAX_LENGTH_FOR_FRD_STRING
];
21
22 al_flush_event_queue(q
);
23
24 int key;
25
26 int shift_key_pressed
;
27
28 int ascii_value
;
29
30 if(filename
[0] != '\0')
31 {
32 printf(" get_filename_screen() : string not zeroed.\n");
33 return QUIT_DUE_TO_PROBLEM
;
34 }
35
36
37 strcat( filename,
".frd");
38
39 //printf(" in get_filename_screen.\n");
40
41 do
42 {
43 if( !al_is_event_queue_empty(q
) )
44 {
45 while( al_get_next_event(q,
&event
) )
46 {
47 switch( event.type
)
48 {
49 case ALLEGRO_EVENT_DISPLAY_CLOSE:
50 return USER_QUIT_PROGRAM
;
51
52 case ALLEGRO_EVENT_KEY_CHAR:
53 key = event.keyboard.keycode
;
54
55 shift_key_pressed
= (event.keyboard.modifiers
& ALLEGRO_KEYMOD_SHIFT
) ?
1 : 0;
56
57 if(key == ALLEGRO_KEY_ESCAPE
)
58 {
59 return USER_QUIT_PROGRAM
;
60 }
61 else if( key == ALLEGRO_KEY_BACKSPACE
)
62 {
63 if( current_character
> 0 )
64 {
65 filename
[current_character
- 1] = 0;
66 current_character
-= 1;
67 strcat( filename,
".frd");
68 }
69 }
70 else if( current_character
< MAX_LENGTH_FOR_FRD_STRING
- 5 && (ascii_value
= get_ascii_value
( key, shift_key_pressed
)) != -1 )
71 {
72 //printf(" here %d \n", (int) ascii_value);
73 filename
[ current_character
++ ] = (char) ascii_value
;
74 filename
[current_character
] = 0;
75 strcat( filename,
".frd");
76 }
77 else if( key == ALLEGRO_KEY_ENTER
&& current_character
!= 0)
78 {
79 return FILENAME_STRING_OKAY
;
80 }
81 break;
82
83 }
84
85
86
87
88 }
89 }
90
91 al_clear_to_color( DARK_RED
);
92
93 al_draw_text( big_font, WHITE,
100,
100,
0,
"Enter a name for the file.");
94
95
96 al_draw_textf( small_font, WHITE,
100,
140,
0,
"Name for file : %s", filename
);
97
98 al_draw_text( small_font, WHITE,
100,
200,
0,
"Press ENTER when filename is finished.");
99
100
101 al_flip_display();
102
103 } while(1);
104
105 return 0;
106}
/* EDIT - sorry it's a lazyish post but I've had a long day and I've started a glass of wine */
/* EDIT 2 */
#SelectExpand
1int get_ascii_value
(int keycode,
int shift_key
)
2{
3
4 if(keycode
>= ALLEGRO_KEY_A
&& keycode
<= ALLEGRO_KEY_Z
)
5 {
6 printf(" returning %d.\n",
(keycode
- ALLEGRO_KEY_A
+ 97) - shift_key ?
32 : 0);
7 return (keycode
- ALLEGRO_KEY_A
+ 97) - (shift_key ?
32 : 0);
8 }
9
10 if( keycode
>= ALLEGRO_KEY_0
&& keycode
<= ALLEGRO_KEY_9
)
11 {
12 return keycode
- ALLEGRO_KEY_0
+ 48;
13 }
14
15 if( keycode
== ALLEGRO_KEY_MINUS
)
16 {
17 if(shift_key
) return '_';
18 else
19 return '-';
20 }
21
22
23 return -1;
24}
Hope I haven't put you off programming 
]]>
