The talks follow failed bids by leaders of the centre-right New Democracy and radical left Syriza bloc.
Correspondents warn Pasok is tainted by its association with unpopular austerity cutbacks.
Sunday's elections revealed a country divided over plans to bring it out of its debt crisis.
Financial chaos has sparked huge social unrest in Greece and
led to a deep mistrust of the once-dominant parties which backed
austerity.
Under the bailout agreement, Athens is due to pass new
austerity measures worth 14.5bn euros (£11.6bn; $18.8bn) next month -
part of cuts required to qualify for bailouts worth a total of 240bn
euros.
'Effort must continue'
After the first two parties failed to find coalition partners,
former finance minister Mr Venizelos will now meet President Karolos
Papoulias to receive the mandate to try to form a government.
"It was clear that in the current stage of this process we
cannot reach a solution but that we must continue this effort," Mr
Venizelos said, according to AP news agency.
"So the mandate I will receive tomorrow will have substance and importance."
But Pasok is now deeply unpopular, says the BBC's Mark Lowen
in Athens - seen as the architects of austerity, and tainted with
allegations of corruption.
It dominated Greek politics for most of the past four
decades, but saw its support slashed on Sunday - coming third with just
41 seats, a quarter of its pre-bailout support.
Its attempt to form a government also appears likely to fail,
our correspondent says, making fresh elections - and weeks of fresh
instability across the eurozone - seem inevitable.
The eurozone's
rescue fund on Wednesday decided to withhold 1bn euros of its latest
instalment of its bailout to Greece pending a meeting of eurozone
finance ministers on Monday.
But the fund said it would disburse 4.2bn euros of the 5.2bn euros due to the country on Thursday.
'Dream' dashed
The fresh coalition bid follows in the wake of a failed attempt by Alexis Tsipras, the leader of Greece's far-left Syriza bloc.
Mr Tsipras said he had failed to reach agreement with
mainstream parties because of his insistence on rejecting austerity
measures demanded by the EU and IMF as part of a bailout deal.
He made the announcement after failed talks with the Pasok
and New Democracy parties, which support the bailout. Talks with the
Communist KKE and smaller left-wing Democratic Left also failed.
He told Syriza MPs: "We cannot make true our dream of a left-wing government."
Earlier on Wednesday, New Democracy leader Antonis Samaras rejected Mr Tsipras's demand to tear up the bailout deal.
Mr Samaras told a party meeting that the proposal would "lead
to immediate internal collapse and international bankruptcy, with the
inevitable exit from Europe".
"[Amending] the loan deal is one thing, it is a completely
different thing to unilaterally denounce it. The second option leads to
catastrophe that is certain and immediate," he said.
If it rejects the deal with the IMF and EU, Athens will be
unable to draw its international loan, meaning it would again face the
prospect of bankruptcy and possible exit from the euro, our
correspondent says.
Previous Greek governments agreed to make deep cuts to
pensions and pay, raise taxes and slash thousands of public sector jobs
in return for the bailouts.
Both Germany and the EU have made clear they expect Athens to honour its commitments.
German Foreign Minister Guido Westerwelle said on Wednesday:
"Germany would like to keep Greece in the eurozone, but Greece's fate is
now in its own hands."
Source: BBC News