In System Generator 2015.2, a known issue exists where the MultAdd block's User defined output data type is always configured to Unsigned.
It can never generate signed output values, regardless of the user defined configuration.
To work around this issue, you can change the "User defined output data type" in the MultAdd block to unsigned and then use the reinterpret block to convert it to signed.
For example, set the MultAdd to have UFix11_10 output and then in the Reinterpret block set it to have an output of Fix_11_10.
The issue has been reported and will be fixed in an upcoming version of the Vivado Sysgen software.