Frequency extracted from MATLAB's $\tt fft$ is not very accurate
MATLAB uses one based indexing, DFTs are zero based, so you have to shift your index value by one.  

The bin index corresponds to the the frequency in units of cycles per frame.  Your freq is in units of cycles per second.  The sample count is in units of samples per frame and the sampling rate is units of samples per second. The bin index calculation should be:

\begin{align}
\mathrm{bin} &amp;= \mathrm{freq} \cdot \frac{N}{f_s}\\
15 &amp;= 10 \cdot \frac{1500}{1000}
\end{align}

Where the units are:

$$\rm \frac{cycles}{frame} = \frac{cycles}{second} \cdot \frac{\displaystyle\frac{samples}{frame}}{\displaystyle\frac{samples}{second}} $$

Bin 15 is in index 16 of MATLAB.  Since this is a whole number of cycles per frame, all your other bin values should be zero.

Hope this helps.

Ced



Follow up

Your calculation should have been:

$$\rm \frac{Frequency\ Bin \times Sampling\ Frequency}{Number\ of\ Samples} = \frac{15 \times 1000}{1500} = 10$$

Bin $-15$, aka bin $1485$, will also be non-zero and the complex conjugate of bin $15$.  Each should have a magnitude of $2.5$.  Your DC bin, aka bin $0$, should have a value of $-6$.  The corresponding MATLAB indexes will be $16$, $1486$, and $1$.