Learn Russian: Мы не можем найти ваш дом.

Because it follows можем (we can). When you use a modal verb like мочь (can) or хотеть (to want), the second verb stays in its infinitive (dictionary) form. It works just like English: we say "We can find" instead of "We can we find".

Why is the verb мочь spelled as можем here?

Мочь (to be able to) is an irregular verb. For the pronoun мы (we), the stem changes to a ж sound, giving us можем. You just have to memorize this shift, as it changes back and forth: for example, я могу (I can) but мы можем (we can).

Where does the negative word не go when using a modal verb and an infinitive?

To say "cannot", не must go directly before the conjugated verb (можем). If you placed it before the infinitive (мы можем не найти), the meaning would change completely to "we might not find [it]".

Since "your house" is the direct object being found, shouldn't ваш дом change its ending?

It actually is in the Accusative case here because it is the direct object! However, for masculine, inanimate (non-living) nouns like дом (house), the Accusative form looks exactly the same as the Nominative (dictionary) form. The matching pronoun ваш (your) follows the same rule and looks unchanged.