Multi-Select Parameter Issues - DataType Also?

I've tried several things to get my multi-select parameter to take more than one value but it seems to only take the 1st one I input. I've been working on this for days and been trying alternate methods but nothing seems to work. I think my first question is if I'm planning on using a parameter as a multi-select parameter what is the data type I should call that parameter. In my case here, the evt_key is unique and will always be 36 characters. I cant imagine in my lifetime that I'm going to have any more than 20 items in the multiselect. Am I looking over something or any other suggestions? I also attached my sp at the bottom.

Thanks

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CREATE procedure dbo.rpt_client_ngcoa_event_registration_attendees_by_state
@evt_key varchar(4000)
as

set nocount on
set transaction isolation level read uncommitted

create table #tmp_dates
(
tmp_evt_key varchar(4000),
tmp_title nvarchar(160),
tmp_start_date datetime
)

create table #temp
(
tmp_key varchar(4000), -- event
evt_title nvarchar(160),
reg_key varchar(38),
reg_session_code nvarchar(400),
type nvarchar(20),
reg_name nvarchar(300),
reg_sort nvarchar(300),
reg_org nvarchar(300),
reg_date datetime,
amount_paid money,
adr_city_state_code NVARCHAR(110) NULL,
adr_state NVARCHAR(40) NULL,
adr_country NVARCHAR(60) NULL,
adr_city NVARCHAR(40) NULL,
adr_post_code NVARCHAR(20) NULL,
src_code NVARCHAR(50) NULL,
days_out int
)

/*
-- FOR MUTLI SELECT FUNCTIONALITY --
create table #tempet
(
tmpet_key varchar(4000)
)

if @evt_key = '' or @evt_key is null

begin
insert #tempet
select distinct evt_key
from ev_event
where evt_key in ('00394886-dfc7-4466-b674-1f2b3ede79ea', '10BE664D-DA4F-44F4-802C-ABD4FC015FB4','56504014-9787-4207-8FAD-EC6C6A384C1D')
and evt_delete_flag = 0
end

else
begin
insert #tempet
exec _selectstringfromstring @idstring = @evt_key,@intorstring='S'
end

if @evt_key = '' goto finalselect
set @evt_key = dbo.av_preprptguidparam(@evt_key,'returnnull')


*/



IF @evt_key='' SELECT @evt_key=NULL





insert into #tmp_dates
select distinct
reg_evt_key,
evt_title,
evt_start_date
from ev_registrant
join ev_event on reg_evt_key = evt_key
--join #tempet on evt_key=tmpet_key
where reg_delete_flag = 0
and evt_delete_flag = 0 --and reg_evt_key = '10BE664D-DA4F-44F4-802C-ABD4FC015FB4'
and (@evt_key is null or (@evt_key is not null and reg_evt_key = @evt_key))
---AND (@evt_key IS NULL OR (@evt_key IS NOT NULL AND reg_evt_key IN (SELECT item FROM dbo.av_SelectStringFromString(@evt_key,','))))


insert into #temp
select tmp_evt_key,
tmp_title,
reg_key,
-- net_prc_code,
case net_prc_code when null then tmp_title
when '' then tmp_title
when 'NULL' then tmp_title
when ' ' then tmp_title
else net_prc_code end,
'Reg Code',
cst_ind_full_name_dn,
cst_sort_name_dn,
cst_org_name_dn,
dbo.av_end_of_day(reg_add_date),
net_payamount,
adr_city_state_code,
adr_state,
adr_country,
adr_city,
adr_post_code,
src_code,
datediff(dd, reg_add_date, tmp_start_date)
from #tmp_dates
join ev_registrant on reg_evt_key = tmp_evt_key
join vw_ac_invoice_detail on reg_ivd_key = net_ivd_key
join co_customer on cst_key = reg_cst_key
LEFT JOIN co_customer_x_address x ON x.cxa_key = reg_cxa_key AND cxa_delete_flag = 0
LEFT JOIN co_address a ON a.adr_key = x.cxa_adr_key AND adr_delete_flag = 0
LEFT JOIN co_source_code o ON src_key = reg_src_key AND src_delete_flag = 0
--join #tempet on reg_evt_key=tmpet_key
where reg_delete_flag = 0
and reg_cancel_date is null
and cst_delete_flag = 0
AND (@evt_key IS NULL OR (@evt_key IS NOT NULL AND tmp_evt_key IN (SELECT item FROM dbo.av_SelectStringFromString(@evt_key,','))))



finalselect:
select *
from #temp
--where (@evt_key IS NULL OR (@evt_key IS NOT NULL AND tmp_key IN (SELECT item FROM dbo.av_SelectStringFromString(@evt_key,','))))
order by days_out desc, type, reg_name
GO