The Smasher is one of the five basic upgrade choices from the Tank. Unlike other Tank Upgrades, it becomes selectable at Level 30. It may further upgrade into the Landmine, the Auto Smasher or the Spike.
It's considered one of the basic upgrades because it would be the first upgrade you make if you chose to wait until level 30. I'm assuming that the diagram above is tiered out to the corresponding levels you need to be to upgrade to that tank (meaning the Smasher tank is in the level 30 tier), but having never played the game, I can't say for sure.
Interestingly, there is a recurring pattern of colors in the upgrade selection menu and the Class Tree. The first tank will always have a light blue background. The second will be green. The third will be red. The fourth will be yellow. The fifth will be dark blue, and the sixth will be purple (This order can be found in tank's add stats buttons which are color-reversed). Regardless of speculation, colors do not seem to represent anything other than a simple order pattern to more easily discern tanks apart when quickly selecting an upgrade
Note that Tank->Twin is light blue, and adds an forward extra barrel, while Machine Gun->Gunner has a similar effect, but in a different color.
It is interesting that tanks which appear in multiple paths (Twin Flank and Quad Tank) have the same color both times, but they have been placed in the same order to keep the pattern.
In the top-left corner, you can select which tank you can upgrade to. As a Basic Tank, you'll most likely see this:
If you reach level 30 as a Twin, you would see a different list of upgrades.
If you press the gray Ignore button below the upgrades, the Upgrades menu will slide out. It will return again if you hover your mouse near the top-left corner. It will also automatically reappear if you reach levels 30 or 45. If it will not appear even when hovering your mouse near the corner, it is probably because you don't have any available upgrades.
Best Answer
Are you level 30 yet? Apparently you need to be.
From the Wiki:
It's considered one of the basic upgrades because it would be the first upgrade you make if you chose to wait until level 30. I'm assuming that the diagram above is tiered out to the corresponding levels you need to be to upgrade to that tank (meaning the Smasher tank is in the level 30 tier), but having never played the game, I can't say for sure.