The numeric_std_unsigned package was added in the VHDL 2008 standard. In order to use functions from this package, your code should comply with the VHDL 2008 syntax, and should be compiledw ith the -2008 switch.
Right-click the design in the Design Manager
Select Properties... from the drop-down menu
Go to Compilation | VHDL | General
In the Standard version drop-down menu, select 1076-2008
Click Apply and <OK>
Please use the -2008 switch with the acom/vcom command. For example:
acom -2008 -dbg -lib work myfile.vhd