Check enable box whitout a mouse on an imac
Depending on which keyboard you have, the answer is different.

Standard Keyboard:

This assumes you have the standard, smaller keyboard shown here:

If this applies to you, then to click using Mouse Keys simply press I.

Extended Keyboard:

This assumes you have the extended, larger keyboard shown here:

If this applies to you, the to click using Mouse Keys simply press the 5 key on the keypad on the right.

Hope this helps!