if (Key.isDown(KEYCODEGOESHERE)) {

Is the condition you should use. Here's a list of the key codes:

Letters:

A
65

B
66

C
67

D
68

E
69

F
70

G
71

H
72

I
73

J
74

K
75

L
76

M
77

N
78

O
79

P
80

Q
81

R
82

S
83

T
84

U
85

V
86

W
87

X
88

Y
89

Z
90

Numbers:

0
48

1
49

2
50

3
51

4
52

5
53

6
54

7
55

8
56

9
57

Numpad (or is it Numbpad? Macromedia seem to think so):

Numbpad 0
96

Numbpad1
97

Numbpad 2
98

Numbpad 3
99

Numbpad 4
100

Numbpad 5
101

Numbpad 6
102

Numbpad 7
103

Numbpad 8
104

Numbpad 9
105

Multiply
106

Add
107

Enter
108

Subtract
109

Decimal
110

Divide
111

Function keys:

F1
112

F2
113

F3
114

F4
115

F5
116

F6
117

F7
118

F8
119

F9
120

F10
121

F11
122

F12
123

F13
124

F14
125

F15
126

Other keys:

Backspace
8

Tab
9

Clear
12

Enter
13

Shift
16

Control
17

Alt
18

Caps Lock
20

Esc
27

Spacebar
32

Page Up
33

Page Down
34

End
35

Home
36

Left Arrow
37

Up Arrow
38

Right Arrow
39

Down Arrow
40

Insert
45

Delete
46

Help
47

Num Lock
144

; :
186

= +
187

- _
189

/ ?
191

\Q ~
192

[ {
219

\ |
220

] }
221

'' '
222