Not sure how that thing works but its doesnt look to complex, assuming you have mapped your key/stick correctly?
So you get 'LA' symbology in the hud?
from the manual for R-73 / R-60 Air to Air missiles.
To deploy a weapon, the pilot needs to execute the following steps:
Detect the target
Lock or designate the target
Deploy weapon
Air-to-Air Weapons
R-73 and R-60 short range missiles
The Su-25T can carry the R-73 and R-60 short range air-to-air missiles in the longitudinal aiming mode. When this mode is activated, the missile’s seeker has a scan zone of 2 degrees that is directed forward along the aircraft’s longitudinal axis. The target must enter this seeker field of view, which is represented by the center of the aircraft symbol on the HUD, to automatically lock the target.
The target lock and launch procedures consist of the following steps:
Step 1
Select air-to-air mode with the [6] key. In either case, longitudinal aiming mode will be activated.
Step 2
Maneuver your aircraft to place the center of the HUD aircraft symbol over the target. When the missile seeker is in lock range, the aiming will jump to the target; the yellow launch authorized lamp will flash; and a lock audio signal will sound. The lock range depends greatly on the target’s IR-signature. The maximum signature for an aircraft is when flying at high-altitude, at full AB, and you are in the target’s rear-hemisphere. Note that helicopters have minimal IR signatures and they may be difficult to acquire. When the missile seeker achieves a lock and the "LA" message is displayed on the HUD, this is only an indication that the target has been locked; it does not mean that the target is within range of the missile. Launching a missile too early may lead to a miss because the missile has insufficient energy to intercept the target. It is recommended that you do not launch until the target’s shape is visible or 2 km.
Step 3
Press the trigger on your joystick or press the [Space] key to launch the missile. The missile is "fire and forget" and requires no additional support from the launch aircraft.
Manual is located by default in this folder just in case you may of missed it.
Program Files\Eagle Dynamics\DCS World\Mods\aircrafts\Su-25T\Doc
If that doesn't work post a screeny of your SU25 control options page showing the key mapping