$-\bar{\Delta}$ stands for the operator closure of $(-\Delta, C^{\infty}_o(\mathbb{R}^3))$, I suppose. If this is the case then you should:
- Apply a Fourier transform to diagonalize $-\Delta$, so that it becomes a multiplication operator in Fourier space;
- Ascertain that this multiplication operator is essentially self-adjoint and determine its domain of self-adjointness;
- By means of an inverse Fourier transform, deduce from this that the domain of $-\bar{\Delta}$ is Sobolev space $H^2(\mathbb{R}^3)$;
- Apply the Sobolev imbedding theorem to conclude that this space is imbedded into a space of bounded and continuous functions (Hölder continuous, actually).
P.S.: I had not seen the second part of the question. For this you need to show that, if $\psi \in H^2(\mathbb{R}^3)$ is such that $V(x)\psi(x)\in L^2(\mathbb{R}^3)$, then $\psi\equiv 0$. Again, use the fact that $\psi$ is continuous and argue by contradiction: if $\psi \ne 0$ then there exists a bounded open subset $\Omega$ of $\mathbb{R}^3$ such that $\lvert \psi(x)\rvert \ge m>0$ for every $x \in\Omega$. From the fact that $V(x)\psi(x)\in L^2$ you infer from this $V\in L^2(\Omega)$, a contradiction.