Code:
I have changed it to work for FM 88-108MHZ.  Of course, you have to make it work for 250KHz, 500KHz and 1MHz steps

'Fmin =88 MHz, F max= 108 MHz 
' ** DECLARATION OF THE VARIABLES

b1 var BYTE
b2 var BYTE
b3 var BYTE
b5 var BYTE

F1 var WORD
F2 var WORD
F3 var WORD


ADDR1 VAR BYTE
TMP VAR WORD
TMP_LO VAR TMP.LOWBYTE
TMP_HI VAR TMP.HIGHBYTE
PLLBASE VAR WORD
PLL VAR WORD
LO VAR PLL.LOWBYTE
HI VAR PLL.HIGHBYTE
PAS VAR BYTE '1=125kHz, 2=250kHz, 4=500kHz, 8=1MHz 

'*** INITIAL DATA *** 

ADDR1=$C2 'adress I2C of SDA5055 (ADDR1=$C0 , adress I2C of TSA5511)
PLLBASE=88/0.125  'beginning with 88 MHz : 88 / 0,125 = 704
PAS = 1 'pas of 0.125 MHZ by default
PAUSE 100
LCDOUT $FE, 1 ' Clear LCD screen
LCDOUT " PLL FM 88-108" ' post text during 1 second
LCDOUT $FE,$C0 
LCDOUT " NIKOS" 
PAUSE 1000 

EEPROM 0,[$90,$01] 'initial data TMP = PLLBASE  corresponds to F = 88 MHz


'The starting point is 88 MHz given by PLLBASE. One adds a shift of 125kHz * TMP
'TMP is on 16 bits, therefore 65535 possible frequencies.
' 
'*** READING OF THE DECALAGE TMP ***

READ 0,TMP_LO 'reads the 8 bits of weak weight
READ 1,TMP_HI 'reads the 8 bits of strong weight


' *** BEGINNING OF THE PROGRAM***

MAIN: 
PLL = PLLBASE + TMP

I2CWRITE SDA,SCL,ADDR1,[HI,LO,$8E] 'Sending of the data to the module 

GOSUB CALCUL 'calculate the frequency for posting
GOSUB AFFICHAGE 'post the frequency
PAUSE 500


'*** PRINCIPAL LOOP ***

BOUCLE: 
BUTTON UP,1,10,2,b1,1,MONTE 'supervise the button UP
BUTTON DOWN,1,10,2,b2,1,DESCEND 'supervise the button DOWN
BUTTON MEMO,1,255,0,b5,1,STORE 'supervise the button MEMO
' only step size of 125KHz is handled
' BUTTON CH_PAS,1,255,0,b3,1, CHOIX 'supervise the button PAS 
PAUSE 100 'pauses of 0.1s
GOTO BOUCLE


MONTE: '** INCREASE THE FREQUENCY OF 1 PAS (STEP)

TMP = TMP + PAS
IF TMP > 160 THEN TMP = 0 'Fmax = 108 MHz
GOTO MAIN

DESCEND: '** DECREASE THE FREQUENCY OF 1 PAS (STEP)

IF TMP IF TMP = 0 THEN TOP
TMP = TMP - PAS
GOTO MAIN

TOP: TMP=(108-88)/0.125   ' max reading in Tmp
GOTO MAIN

RAZ_TMP: TMP = 0
GOTO MAIN

CHOIX: '** CHOICE OF THE PAS (STEP) 

IF PAS = 8 THEN SAUTE
PAS = PAS*2
GOSUB AFFICHE_PAS
GOSUB AFFICHAGE 'post the frequency
GOTO BOUCLE

SAUTE: PAS = 1
GOSUB AFFICHE_PAS
GOTO BOUCLE

CALCUL: ' ** CALCULATION OF THE FREQUENCY

F1= PLL-800 'cut off 1GHz 
F2=F1/8 'give them MHz
F3=125*(F1//8) 'give them kHz
RETURN

AFFICHAGE: 
LCDOUT $FE, 1 'Clear LCD screen 
LCDOUT #1 'post the 1 of Giga
LCDOUT #F2 'post the other digits 
LCDOUT "." 'post the comma
LCDOUT #F3 'post the Khz
LCDOUT " MHz "