You can use Thrusters/Dash by pressing the "Thrusters/Dash" key (default key: Shift) while holding one or more directional keys (default keys: W/A/S/D).
Inability to use Thrusters/Dash by performing the above may be related to a limit in how many keys can be pressed simultaneously on your keyboard (a.k.a. "N-Key Rollover").
Since pressing multiple keys at once is of critical importance in GUNDAM EVOLUTION, we suggest you visit the webpage of your keyboard's manufacturer and check its specifications.
Should none of the above solve the issue, please contact us through the "Contact" page and provide us with a detailed description pertaining to your situation.