You need to have the Native Spiren TestCenter-API loaded before you can load the HLTAPI, so you need to set the packageIndex.tcl to point to both API locations.
Example:
set STC_INSTALL_DIR "C:\\Program Files\\Spirent Communications\\Spirent TestCenter 3.50\\Spirent TestCenter Application"
package ifneeded SpirentTestCenter 3.31 [list source [file join $STC_INSTALL_DIR SpirentTestCenter.tcl]]
package ifneeded stc 3.31 [list source [file join $STC_INSTALL_DIR SpirentTestCenter.tcl]]
package ifneeded stclib 3.31 [list source [file join $STC_INSTALL_DIR stclib.tcl]]
set hltdir "C:\\HLTAPIStuff\\HLTAPI340\\SourceCode"
package ifneeded SpirentHltApi 3.00 [list source [file join $hltdir hltapi_5.10_stc_2.10.tcl]]