How to create a ROM file?

yep, I've copied and pasted code from there...
CHGET: .EQU $9F
JIFFY: .EQU $FC9E
RSLREG: .EQU $138
EXPTBL: .EQU $FCC1
BOTTOM: .EQU $FC48
HIMEM: .EQU $FC4A
SLTWRK: .EQU $FD09
ENASLT: .EQU $24
LDIRVM: .EQU $5C
CHGMOD: .EQU $5F
SCRMOD: .EQU $0CAF
CHSTART: .EQU $E5C0
COLSTART: .EQU $E5C2
LINECOUNTER: .EQU $E5C4
DBFLAG: .EQU $E5C5
NAMETABLE: .EQU $E5C6
.ORG $4000
.DB $AB
.DW BEGIN
.DW $0,$0,$0,$0,$0,$0
BEGIN:
CALL RSLREG ;read primary slot #
RRCA ;move it to bit 0,1 of [Acc]
RRCA
AND %00000011
LD C,A
LD B,0
LD HL,EXPTBL ;see if this slot is expanded or not
ADD HL,BC
LD C,A ;save primary slot #
LD A,(HL) ;See if the slot is expanded or not
AND $80
OR C ;set MSB if so
LD C,A ;save it to [C]
INC HL ;Point to SLTTBL entry
INC HL
INC HL
INC HL
LD A,(HL) ;Get what is currently output
;to expansion slot register
AND %00001100
OR C ;Finally form slot address
LD H,$80
JP ENASLT ;enable page 2
START:
LD A,7
LD (LINECOUNTER),A
LD HL,46007
LD (CHSTART),HL
LD HL,46503
LD (COLSTART),HL
LD HL,58112
LD (NAMETABLE),HL
LD A,0
LD (DBFLAG),A

Not quite. But before my answer I would like you to read this thread, because it alredy contains a much better explanation than I could possibly produce.

A BINary file contains a 7-byte header used by BLOAD command on BASIC. A ROM file should contain a number of other informations, meant to BIOS properly detect that there's a cartridge connected to that slot and a number of pointers to allow cartridge (which could be a software or a hardware with BIOS drivers that could be used by BASIC) to work properly. I couldn't find a Wiki article with this information, so:

DEFB "AB" ; expansion ROM header
DEFW initcode ; start of the init code, 0 if no initcode
DEFW callstat; pointer to CALL statement handler, 0 if no such handler
DEFW device; pointer to expansion device handler, 0 if no such handler
DEFW basic ; pointer to the start of a tokenized basicprogram, 0 if no basicprogram
DEFS 6,0 ; room reserved for future extensions

Also, a ROM file shoud have exact size; meaning: if your program was meant to fit on a 16k cartridge, your ROM file should be padded to exactly 16384 bytes.

And if your program is bigger than 16k, there are a few other measures to be taken (on BASIC you have a bunch of ~16k files which contain loader program along the code to switch slots and copy the program to the right addresses within $4000~$7FFF area, and when you assemble a ROM file -either to run on emulator or to burn to a cartridge- you wouldn't need this loader, you can just place the code on the right order).

First problem I saw:

	.DB	$AB

This should be:

	.DB	"AB"

or...

	.DB	$41,$42

One more thing. I'll fix It, then compile a bin file and then rename it as rom?

MRC too.
https://www.msx.org/wiki/The_Memory
https://www.msx.org/wiki/Slots
Program sample to make a ROM:
https://www.msx.org/wiki/Assembler_for_Dummies_%28Z80%29#Pro...
This documentation is valid for all MSX generation.