ModelSim-Altera License ERROR

I've installed Quartus II 15.0, and ModelSim on my Ubuntu Linux 14.04 LTS and I keep get the same error messgae when I want to simulate my design with RTL simulation. The message says that in the file:://path/to/project-folder/project-name_nativelink_simulation.rpt I can read more details about the error. and in this file it's written:

"Can't launch ModelSim-Altera Simulation software -- make sure the software is properly installed and the environment variable LM_LICENSE_FILE or MGLS_LICENSE_FILE points to the correct license file."

I really need to simulate for my project and I want to learn use ModelSim but I can't focus on the design due to these errors...

The full text of this .rpt file is attached here:

Info: Start Nativelink Simulation processInfo: NativeLink has detected a mixed Verilog and VHDL design -- verilog simulation models will be used
========= EDA Simulation Settings =====================
Sim Mode              :  RTL
Family                :  cycloneive
Quartus root          :  /path/to/Quartus/15.0/instalation/quartus/linux64/
Quartus sim root      :  /path/to/Quartus/15.0/instalation/quartus/eda/sim_lib
Simulation Tool       :  modelsim-altera
Simulation Language   :  verilog
Simulation Mode       :  GUI
Sim Output File       :  
Sim SDF file          :  
Sim dir               :  simulation/modelsim
=======================================================
Info: Starting NativeLink simulation with ModelSim-Altera software
Sourced NativeLink script /path/to/Quartus/15.0/instalation/quartus/common/tcl/internal/nativelink/modelsim.tcl
Error: Can't launch ModelSim-Altera Simulation software -- make sure the software is properly installed and the environment variable LM_LICENSE_FILE or MGLS_LICENSE_FILE points to the correct license file.
Error: NativeLink simulation flow was NOT successful
================The following additional information is provided to help identify the cause of error while running nativelink scripts=================
Nativelink TCL script failed with errorCode:  issued_nl_message
Nativelink TCL script failed with errorInfo:  Can't launch ModelSim-Altera Simulation software -- make sure the software is properly installed and the environment variable LM_LICENSE_FILE or MGLS_LICENSE_FILE points to the correct license file.
    while executing
"error "$emsg" "" "issued_nl_message""
    invoked from within
"if  {
        set emsg "Can't launch $tool Simulation software -- make sure the software is properly installed..."
    (procedure "launch_sim" line 89)
    invoked from within
"launch_sim launch_args_hash"
    ("eval" body line 1)
    invoked from within
"eval launch_sim launch_args_hash"
    invoked from within
"if  {
                set savedCode $::errorCode
                set savedInfo $::errorInfo
                error $result $savedInfo $savedCode
            } else {
            ..."
    invoked from within
"if  {
            set status 1
            if  {
                set savedCode $::errorCode
                set sav..."
    (procedure "run_sim" line 78)
    invoked from within
"run_sim run_sim_args_hash"
    invoked from within
"if  {
            set savedCode $::errorCode
            set savedInfo $::errorInfo
            error "$result" $savedInfo $savedCode
        } else {
            er..."
    (procedure "run_eda_simulation_tool" line 336)
    invoked from within
"run_eda_simulation_tool eda_opts_hash"